Anyone can top a brown-iced sheet cake with a monster truck toy, but an edible monster truck made entirely from cake is sure to rev up the fun at a kid's birthday party. Three-dimensional cake pans shaped like monster trucks make the task easier, but you can also cut a layered, rectangular cake into a truck shape. Bake and shape the cake, and then preserve it in your freezer until it's time to decorate. It helps to use a monster truck toy or picture as inspiration; ask the birthday celebrant about his favorite truck to get the color scheme correct.
Preparing for Decoration
Bake several rectangular cake layers about 1 inch thick, baking as many as you need to achieve the desired height for the monster truck. Allow the cakes to cool completely. You can bake the cakes in loaf pans or sheet pans, depending on the monster truck size you want. Instead of baking individual cakes 1 inch thick, you can cut a thicker cake into 1-inch layers with a serrated knife or a piece of fishing line. Mark the 1-inch height with toothpicks along the sides of the cake. Line up the fishing line with the toothpicks and use a sawing motion to begin cutting the cake. Pull the fishing line through, using the toothpicks as a guide, to cut clean, even layers.
Spread a thin layer of buttercream frosting -- about 1/4 to 1/2 inch thick -- over the top of each cake layer. Stack the cake layers on top of each other with the sides lined up as evenly as possible. The buttercream frosting acts as a glue to bind the cake layers and also adds flavor throughout the cake. Don't apply frosting to the outside of the finished layer cake. Place the layer cake in the freezer for about 1 hour or until partially frozen, so the cake is easier to cut and shape.
Cut out a basic truck shape from the partially frozen cake with a long, serrated knife. Cut out the truck bed, leaving the cab protruding higher than the bed and hood. Make a steep sloping cut from the truck hood to the top of the cab for the windshield. Cut four half-circle indentations along the bottom for wheel wells -- use a spoon to scoop out the cake for the wheel wells. Place the cake back in the freezer to return it to a partially frozen state.
Cut a final 1- to 1 1/2-inch-thick slice of cake for the base, making it at least 1 inch smaller than the bottom of the truck. This piece of cake doesn't require frosting because it won't be seen, but you might cover it with brown or chocolate frosting, if desired. Set this piece in place on the cake board and place the truck-shaped layer cake, centered, on top.
Spread a thin layer of plain buttercream frosting over the entire cake with an icing spatula to seal in the crumbs. For best results, use a crusting buttercream made with vegetable shortening in place of half or all of the butter in the recipe. Allow the cake to rest for about 30 minutes until the buttercream forms a hardened crust. Lay a nontextured paper towel on the frosting and smooth over it with your hands to smooth out any bumps or ridges in the frosting, making the cake ready for decoration.
Divide the remaining buttercream frosting into small bowls and tint it with your choice of food colorings for the monster truck. Dip a toothpick in food coloring gel and swirl it into the frosting. Stir the frosting until the color is thoroughly mixed without streaks. You'll need more frosting for the body of the truck than for smaller details, such as windows or the truck's bumper. Dark gray or yellow works well for windows and windshields, while you might use a lighter gray for the bumper and grill details.
Decorating the Truck Body
Spread buttercream icing over the windshield and window area until completely smooth. Don't worry about getting the shapes perfect, because you will outline these features later. Use an angled icing spatula so you can access these areas without hitting other parts of the truck with the handle, which would happen with a straight spatula or butter knife. Keep a glass of hot water next to your work station to help you spread the icing smoothly. Dip the spatula in the hot water to heat the blade, and wipe off the water. The warm blade melts the sugar and fat in the frosting slightly so it spreads smoothly.
Outline the windows and windshield with icing using a piping bag fitted with a medium-sized round tip. Use the same color frosting as you plan to use for the truck body. Squeeze the bag with firm, steady pressure for an even line. Move the tip slowly to follow the shape of the windows without breaking the icing stream.
Spread a layer of light gray frosting in a rectangle across the front of the truck for the grill. Drag a fork through the frosting to create the lines that define the grill. Alternatively, pipe several thin, straight lines of light gray frosting with a piping bag and a small, round decorating tip. Outline the grill with black frosting and a medium, round decorating tip. You can even draw a small emblem in the center of the grill to indicate a certain truck model, or write the child's age instead.
Spread two smooth squares of yellow frosting to make headlights at the front of the truck. Outline the squares with yellow frosting from a piping bag with a small, round decorating tip.
Spread black icing inside the wheel well indentations, using either an offset spatula or the back of a spoon.
Fit a piping bag with a small star decorating tip and fill the bag with the frosting for the truck body. Fill in the entire truck body with tiny stars, using the outlines for the windows, grill and headlights as guides. Hold the tip so it barely touches the cake and squeeze the bag gently to squeeze out a star. Stop squeezing and lift straight away from the cake to drop the star shape. Reposition the bag directly beside the previous star and make a second star, repeating this pattern in rows until you've filled in the truck body. If you want the wheel wells to bump out from the truck body, apply several layers of stars on top of each other to add dimension.
Measure the front of the truck just below the grill and cut a sugar wafer cookie or pretzel rod to this length. Squeeze a bead of frosting on one side of the cookie or pretzel piece -- this acts as glue -- and press it onto the cake below the grill to create the truck's bumper. Cut two short cookie or pretzel pieces and attach them to either side of the truck to make the sides of the bumper. Cover the bumper piece with black or dark gray frosting applied with a spatula or large, round decorating tip. You can create various lines with round tips to mimic the look of a real bumper. Repeat this process to make the rear bumper. You might need to trim the width of the cookie or pretzel to custom fit the size of your truck.
Spread a layer of black frosting around the outside edges of four chocolate doughnuts appropriate for the truck size. Position the tines of a fork at the center of the frosting and drag it toward the edge at a diagonal. Reposition the fork at the center, dragging it in a diagonal line toward the opposite edge to create the look of tire treads. Set the doughnuts aside while the frosting hardens. Use melted chocolate in place of frosting, if preferred.
Squeeze a bead of frosting in a circle around one face of each of the doughnuts. Press this side of each doughnut against the wheel wells to hold the doughnut tires in place. If the doughnuts don't stay in place with frosting alone, push small dowels, toothpicks or straws through them and into the truck body.
Draw a square of four thick black lines on the roof of the monster truck to mimic the look of roof racks. Use a large, round decorating tip. While optional, you can also make roll bars on the bed of the truck. Draw a square around the back of the truck cab. Set pretzel sticks at a diagonal extending from the top of the truck cab down to the pickup bed sides. Squeeze black frosting over the pretzel sticks.
Squeeze a dot of frosting onto the sides of three or four miniature marshmallows. Press these onto the roof of the monster truck with the round ends facing front to back, giving the look of cab lights. The end marshmallows should line up with the black lines along the sides for the roof racks; place the marshmallows centered on the front line for the roof racks.
Write the birthday child's name across the hood of the truck with a medium, round decorating tip. Draw a symbol or write a number on the sides of the truck, if desired.
When serving a large party, you can display the monster truck cake on top of a large sheet cake instead of a cake board. Decorate the sheet cake to look like a monster truck arena before setting the base and monster truck cake in place. You can cut out bits of a flat cake to give the appearance of hills, frost the sheet cake brown to resemble a dirt track, and add crumbled cookies to look like real dirt. Try placing clean toy cars on the sheet cake so it looks as though the monster truck cake is crushing the cars.
Put the birthday candles on the monster truck bed.