Bake your Funfetti cake in a 9x13 baking pan according to the package directions and allow the cake to cool.
Using 9-ounce dessert cups (approximately 3.5 inches across), turn the cups upside down and firmly press them into the cooled cake, twisting slightly to cut out circles. You should be able to cut 6 full circles from the cake, then use the remaining scraps to create 2 additional portions. Depending on the size of your cups, the final yield may vary.
Press each cake circle down to the base of the cup, and spoon or pipe your vanilla frosting on top to fill the entire cup. Level the frosting on the top to create a flat surface, then pour rainbow sprinkles on a shallow bowl or plate. Dip the frosting into the sprinkles and press down lightly, but with just enough pressure to make sure they stick well.
Carefully invert the cake onto the sprinkles so the frosted side is facing down.
Gently press the cake into the sprinkles to coat the entire surface.
Lift the cake away from the sprinkles and place onto a serving board.
Notes
One 16-ounce tub of frosting covered 8 cakes for us.
Use Cooled Cake: Letting your cake cool fully will make it much easier to decorate and frost.
Use Even Pressure: Press the dessert cup gently to create consistent dot shapes.
Work Quickly: Once the frosting is spread, press into sprinkles before the frosting begins to harden.
Choose Fine Sprinkles: Nonpareil sprinkles or classic rainbow sprinkles create the best coverage and texture.
Save the Cake Scraps: Crumble the leftover cake pieces into dessert cups, then layer with frosting and rainbow sprinkles for more individual mini dot cakes. It's a fun way to enjoy every last bite while avoiding food waste.
Room Temperature: Best served the same day for the most vibrant appearance.
Refrigerator: Store covered for up to 1 day, but some color bleeding from the sprinkles may occur.
Make Ahead: Bake the cake ahead of time, but decorate as close to serving as possible.