Black forest cake is traditionally prepared by alternating layers of chocolate cake with whipped cream, sour cherries and chocolate shavings. You can use whipped cream or whipped topping as frosting for your black forest cake, but these toppings are often not thick enough to hold the shape of any designs you pipe onto the cake. A thicker whipped cream-based icing will provide the stability necessary to decorate your black forest cake.
Place a large mixing bowl and the beaters from your electric mixer into the freezer until these items are cold to the touch.
Pour 2 cups of heavy cream and 1/4 cup confectioner's sugar into the cold mixing bowl. Whip the mixture with your electric mixer and the cold beaters until soft peaks form. This process will take several minutes.
Add 1/4 cup piping gel and 1 tsp. clear vanilla extract to the icing when it has reached the soft peak stage. Continue to beat the mixture just until it has formed stiff peaks. Stop mixing as soon as you reach the stiff peak stage; otherwise, the icing could separate and become lumpy.
Frost the cake immediately or refrigerate the icing for later use. You will need to keep the frosted cake refrigerated to prevent the icing from melting and becoming runny.
Spoon the remaining icing into a parchment bag equipped with a star or open decorating tip. Pipe mounds of icing onto the top of the cake, and top each mound with a cherry if desired.
Piping gel is available in cake decorating supply stores and some craft stores. Clear vanilla extract will flavor your icing without adding color so that the icing remains white. You can use regular vanilla extract if off-white icing is acceptable.