Items you will need
2 boxes cake mix
10-by-15-inch baking pan
T-shirt cakes can be decorated to look like a child's favorite top or a teammate's jersey, or even to say something clever that reflects the birthday girl. If you don't have a pan shaped like a T-shirt, you may be wondering how you could cut a cake to make sleeves. If you bake the cake in a large enough pan, you will be able to cut out the shape you want and use the extra cake to lengthen the shirt.
Mix the cake mix in a large bowl, following the directions on the box.
Preheat your oven to 350 degrees Fahrenheit.
Grease your 10.5-by-15.5-inch baking pan and pour the cake batter into the pan. Place the pan in the preheated oven and bake the cake for 35 to 40 minutes.
Use oven mitts to take the cake out of the oven and let it cool for about half an hour.
Either print and cut out a T-shirt template (see Resources 1) or draw and cut out your own template on a large sheet of paper.
Place the cake on a large flat platter once it has cooled. Lay the T-shirt template over the cake and cut around the lines carefully using a sharp chef's knife.
Remove any excess chunks of cake from the platter. You can either set these aside or add them to the length of the T-shirt. Spread icing over the cake so that any extra chunk additions will blend in with the rest of the cake.
Decorate the cake how you want it.
Be careful when taking the cake out of the oven: both the oven and the baking pan will be extremely hot.