Prepare the batter for the cake's numbers according to the recipe instructions. Line a 13-by-5-inch loaf pan with aluminum foil, spritz it with a little nonstick cooking spray, and pour in the batter.

Cool the cake in the pan for 10 minutes, then remove it and allow it to cool completely on a cooling rack. Cut the domed top off the cake and 1/4 inch off each end. Slice the cake into 13 equal slices (they'll each be 1 inch thick), and freeze the slices for 1 hour.

Remove from freezer, then cut out numbers from each slice using small (1 3/4-inch) cookie cutters. To remove the cake from the cookie cutters, use a wood dowel to gently push the pieces out.

Create a second batch of cake batter (this time, a white cake) according to the recipe instructions. Equally divide the batter among 7 bowls. Using food coloring, color one bowl purple, one royal blue, one sky blue, one green, one yellow, one orange and one red.

At this point, you can preheat the oven to 350 degrees F again. Line a 13-by-5-inch loaf pan with aluminum foil sprayed with nonstick cooking spray.

Pour each colored batter into a disposable pastry bag or large zip-top bag, with a corner snipped off.

Remove the cake numbers from the freezer. If any of them have holes (like the 6 here), pipe in two different batter colors, filling the hole two-thirds full. Then, pipe some squiggles of purple batter into the bottom of the pan, leaving room for other colors in between the squiggles. Pipe another color on top, then another and another, until you've piped all 7 colors into the bottom of the pan in a thin layer.

Prepare the marshmallow creme frosting and frost the cake according to the recipe instructions. Then, you can move on to the rainbow glaze.

Combine 3 ounces finely chopped white candy melts, 3 tablespoons heavy whipping cream and 1 1/2 tablespoons corn syrup in a microwave-safe bowl. Microwave on high in 15-second increments, stirring after each, until melted. Divide the glaze equally among 7 small bowls. Color one purple, one royal blue, one sky blue, one green, one yellow, one orange and one red.