You'll need 2 8-inch round cake pans, 1 6-inch-diameter oven-safe saucepan, 1 clean 28-oz tomato can (label removed) and 1 cupcake pan. Line the bottoms of the cake pans, saucepan and tomato can with parchment and coat with cooking spray. Use 2 ½ cups cake batter in each cake pan, 2 cups in the saucepan and 1 ½ cups in the tomato can. Fill a cupcake paper in the cupcake pan half full of batter.
Bake cakes at 350ºF until tops spring back when touched. Cupcake with bake in about 18 min, 8-inch cakes in about 30 min and the tomato-can and saucepan cakes in 40 to 45 min. Allow to cool and remove from pans.
Wrap cooled cakes in plastic wrap and chill 5 hr or overnight. When you're ready to decorate, remove parchment, trim domed tops and slice each cake into two layers.
Reserve about 3 cups buttercream for assembling cake layers. Divide remaining frosting among 10 bowls (I used about 1 cup each for the purple, blue and teal, ¾ cup each for the greens and yellow, and ½ cup each for the reds, orange and pink) and tint with food coloring.
Assemble cake starting with the 8-inch layers. Spread a bit of jam and a little of the white buttercream on each layer. Stack the 6-inch layers and the layers from the can on top, repeating the jam and buttercream. Place cupcake on top and coat the entire cake with a thin layer of buttercream, smoothing the sides to create uniform edges. Chill until cold, 20 to 30 min.
Transfer cake to a serving platter to decorate. Place colored buttercream in piping bags fitted with a coupler and a #12 plain tip (I bought multiple tips but you can also wash one between colors). Starting at the base with purples and blues, pipe dots around the entire cake in rows. Finish with reds and pink on top.
Keep cake chilled until ready to serve. When buttercream is cold, cake can be loosely wrapped in plastic and will keep, chilled, for up to 2 days.
Don't have graduated, layered cake pans? Explore your kitchen and pantry for various cans and pans.