Preheat oven to 350 degrees Fahrenheit. Glaze two 9-inch pans with cooking spray, and line them with parchment paper. Mix 1 cup cocoa powder and 1 1/2 cups boiling water in a bowl. Combine 2 1/2 cups flour, 2 cups sugar, 1 1/2 teaspoons baking powder, 1 teaspoon baking soda and 1 teaspoon salt in a separate bowl. Add 3 eggs, 3/4 cup vegetable oil, 1/2 cup sour cream and 2 teaspoons vanilla extract, and beat thoroughly. Stir in the cocoa and water. Separate the batter into two pans, and bake for 30 to 40 minutes. Let stand on racks for 10 minutes. Use a knife to push the cakes out of the pans, then turn them over and discard the parchment paper. Frost with 2 1/2 cups chocolate frosting once the cakes are cool.
Gather 11 Kit Kat bars, and break them into individual pieces. Press the bars into the frosting so that they form a circle around the cake.
Pour 1 1/4 cups Reese's Pieces candy into the center of the cake. Wrap a ribbon around the cake if desired.