Place each cake round on a 9” cake cardboard, and use a serrated knife to slice them in half, so you have 8 cake rounds. Place one cake round, on a cake cardboard, on a cake turntable. Top it with a generous cup of frosting, and use an offset spatula to smooth the frosting out to the edge of the cake. Add a second cake round on top.
Place a generous cup (about 7 oz) of frosting in a separate small bowl, and add a bit of yellow food coloring to it, to make a nice light yellow shade. Use this light yellow on top of the second cake round. Add a third cake round, and again separate out a cup of frosting and tint this one a darker shade of yellow (you can do this in the same small bowl you used before, no need to wash more dishes!) Continue to build your cake in this way, gradually transitioning the frosting colors from yellow to yellow-orange to dark orange each time you add a layer of frosting, until you’ve used all 8 layers of cake.
Once the final layer is on, spread a very thin layer of white frosting all around the top and sides of the cake, to seal in any crumbs and blend in any of the yellow/orange frosting colors from the layers inside. Refrigerate the cake for at least an hour, until it’s firm.
While you wait for the cake to firm up, prepare your parchment stencil. Print out or draw your chosen design. Make sure that it will fit on top of your cake and that it’s not too complicated—simple shapes work best. Trace it onto your parchment, and cut out the parchment shape.
When the cake is firm, use the remaining white frosting to frost the top and sides of the cake, to make a smooth, even layer. Use a metal offset spatula to make the top and sides even. It doesn’t have to be perfect, because you’ll be covering it with sprinkles, but the top especially should be as smooth as you can get it, since some of it will be exposed due to the stencil.
Divide your sprinkles into separate bowls. Place the cake on a smaller cake pan, bucket, or anything that elevates it and allows you access to work on the bottom of the cake unencumbered. Place the cake, on the pan, on a baking sheet, to help with clean-up.
Start with your white sparkling sugar, and gently press the sprinkles into the fresh buttercream, coming about 1 inch up the sides of the cake. Use the lightest pressure you can so that you don’t leave finger indentations. Switch to a light gold or yellow color (or, if you don’t have a wide variety of colors, mix together white and yellow to make it lighter) and add another inch of sprinkles above the white. You should be almost tossing them onto the cake, the pressure is so light, not firmly pressing them. Continue up the sides of the cake, gradually transitioning from white to yellow to yellow-orange to orange sprinkles. Make sure the top inch of the sides is your orange color.
Take your parchment stencil, and lay it in the center of the top of the cake. Once positioned, lightly press it down with your fingertips to make sure the whole surface is touching the buttercream, paying special attention to the edges. Don’t press down hard—you don’t want to leave finger indentations—just gently rub it to get it to adhere. Sprinkle your orange sanding sugar all over the top of the cake, going out to the edges where it will meet the sprinkles on the sides.
Refrigerate the cake for at least 1 hour, until the frosting is totally firm. Remove the stencil by using a knife or toothpick to pry up one corner of the parchment, then gently peel it back, showing the white buttercream underneath. As long as the buttercream is firm when you peel it off, the frosting should have a nice and smooth texture. If there are any stray sprinkles on the buttercream, use a soft paintbrush to remove them.
Finally, finish by pressing Sixlets or M&Ms around the bottom of the cake. If you have trouble getting them to stick, pipe a thin layer of buttercream around the bottom and press the candies into that. For the best taste and texture, serve this cake at room temperature!