Birthday cakes should have that little something extra. With colorful sprinkles and the combination of both cake and cookie, this is a dessert that’ll surely impress your guests—including the guest of honor!
Preheat the oven to 350 degrees F. Line three 9-inch round cake pans with parchment paper and grease the sides.
For the chocolate chip cookie layer: Using a stand mixer fitted with the paddle attachment, beat the butter with the brown sugar and granulated sugar for 3 to 4 minutes on medium speed or until light and fluffy. Beat in the egg and vanilla. Add the flour, cornstarch, and baking soda and beat until well combined. Stir in the chocolate chips. Press the dough into an even layer in the bottom of one of the cake pans. Set aside.
For the Funfetti layers: Make the Funfetti cake according to the package directions. Pour half of the batter into a cake pan. Mix the hot fudge sauce with the remaining batter and pour into the remaining cake pan.
Bake the cookie layer for 18 to 20 minutes or until the edges are slightly golden, and bake the Funfetti layers according to the package directions. Let the layers cool in the pans until just warm. Turn the layers out onto a wire rack, remove the parchment, and let cool completely.
Set the cookie layer on a serving plate and spread some of the frosting over the top. Set the white Funfetti layer on top and spread more frosting over the cake, then top it with the chocolate Funfetti layer and spread more frosting on top. Decorate with sprinkles. Slice and serve.
