Cut into this melting rainbow birthday cake to reveal the surprise hiding inside. Each slice of brightly colored tie-dye cake shows the birthday girl or boy's age. The number is created out of dark chocolate cake, which is a wonderful contrast to the rainbow colors. The exterior of the cake is as pretty and colorful as the inside. It's covered in fluffy white frosting and has a rainbow of white chocolate ganache glaze dripping down the sides.
A 13-by-5-inch loaf pan, 1 3/4-inch number cookie cutters, a wooden dowel, disposable pastry bags or large resealable plastic bags and a cake board
For the chocolate cake numbers: Preheat the oven to 350 degrees F. Line a 13-by-5-inch loaf pan with a strip of aluminum foil. Spray the pan with nonstick cooking spray. Combine the dark chocolate cake mix, chocolate pudding mix, milk, sour cream, oil and eggs in a mixing bowl. Using an electric mixer, beat on low speed for 30 seconds, then on medium speed for 2 minutes. Pour the cake batter into the prepared pan. Bake until a toothpick inserted in the center of the cake comes out clean, 52 to 58 minutes. Cool the cake in the pan for 10 minutes, then remove and set on a cooling rack to cool completely. Cut the domed top off the cake and about 1/4 inch off each end. Slice the cake into 13 equal slices. Freeze slices for 1 hour. Remove the cake from the freezer, set on a cutting board, and cut using small 1 3/4-inch number cookie cutters. To remove the cake from the cookie cutters, use a wood dowel to gently push the pieces out. To make a Sweet 16 cake, cut a 1 and 6 out of each cake slice. Line a baking sheet or cutting board with a nonstick silicone mat, nonstick foil or parchment paper. Set the cake numbers on top, cover, and freeze for at least one hour. For the tie-dye cake: Once the cake numbers are frozen, make the colorful cake batter. Preheat the oven to 350 degrees F. Line a 13-by-5-inch loaf pan with a strip of aluminum foil. Spray the pan with nonstick cooking spray. Combine the white cake mix, white chocolate pudding mix, milk, sour cream, oil and egg whites in a mixing bowl. Using an electric mixer, beat on low speed for 30 seconds, then on medium speed for 2 minutes. Equally divide the batter among 7 bowls. Color one bowl purple, one royal blue, one sky blue, one green, one yellow, one orange and one red using the food coloring. Pour each colored batter into a disposable pastry bag or large resealable plastic bag. Snip the tip off each bag. Pipe squiggles of purple batter into the bottom of the prepared pan, leaving room for other colors in between the squiggles. Pipe another color on top, then another and another. Pipe all 7 colors on the bottom of the pan in a thin layer. Remove the cake numbers from the freezer. If any of the numbers have holes in them, pipe in 2 different batter colors, filling the hole two-thirds full. Pipe a very thin amount of batter up against one short side of the loaf pan. Press the cake numbers up against the batter. Press another cake number up against the first number and continue adding cake numbers until you have a row of them going down the length of the entire pan. If using more than 1 number, pipe squiggles of colored batter in between the numbers, alternating colors as you go. Pipe squiggles of colored batter around the outside of the numbers, alternating colors as you go. Pipe squiggles of colored batter on top of the numbers, alternating colors as you go. Smooth out the top of the cake. Do not swirl the colored batter in the pan. Tap the pan on the counter 5 times to help remove big air bubbles. Bake the cake until the top is deep brown and a long toothpick inserted in the center of the cake comes out clean, 55 to 60 minutes. Cool the cake in the pan for 10 minutes, then remove and set on a cooling rack to cool completely. Cut the domed top off the cake and about 1/4 inch off each end. Trim off a thin layer from each long side of the cake. Optionally, freeze the cake until firm. Set the cake on a cake board that is cut 1/4 inch larger than the loaf on all 4 sides. Mark one end of the cake board. Set the cake so the numbers face forward on the marked end of the cake board. For the frosting: Beat the confectioners' sugar, shortening, whipping cream, vanilla and marshmallow creme until light and fluffy. Frost the cake.
For the rainbow-colored glaze: Combine the finely chopped candy melts, whipping cream and corn syrup in a microwave-safe bowl. Heat on high in 15 second increments, stirring after each, until melted. Evenly divide the glaze among 6 small bowls. Color one purple, one royal blue, one sky blue, one green, one yellow, one orange and one red. Allow the glaze to thicken slightly. Create a melting rainbow by pouring colored glaze down the side of the cake, starting with the purple, then royal blue, sky blue, green, yellow, orange and red. Set the cake on a serving dish so you know which end has the numbers facing forward. Cut into the cake and reveal the surprise hiding inside.