Preheat the oven to 350 degrees Fahrenheit. Grease and flour a 9×13 inch baking pan and line it with parchment paper.

Whisk together the flour, baking powder, baking soda, and salt in a medium bowl. Set aside.

Beat the softened butter and granulated sugar on medium speed for 4 to 5 minutes until pale, light, and fluffy. Add the eggs one at a time, beating after each addition, then mix in the vanilla extract.

Alternate adding the flour mixture and milk to the butter mixture on low speed, beginning and ending with flour. Fold in the rainbow jimmies by hand.

Pour the batter into the prepared pan, smooth the top, and bake for 25 to 30 minutes until a toothpick inserted in the center comes out clean. Cool completely.

Beat the softened butter for 3 minutes until pale. Gradually add sifted powdered sugar, then add the heavy cream, vanilla, and salt. Beat on medium-high for 3 minutes until light and fluffy. Transfer to a piping bag.

Use a small round cutter or shot glass (about 2.5 inches diameter) to punch circles from the cooled sheet cake.

Place a cake scrap at the bottom of each cup, pipe a thin layer of buttercream, add a full cake round, then pipe a generous smooth dome of buttercream over the top, slightly above the rim.

Press each frosted top face-down into a shallow bowl filled with rainbow nonpareil sprinkles. Press gently until the frosting is fully coated with a solid blanket of sprinkles.

Refrigerate the assembled dot cakes for at least 30 minutes before serving. Serve cold with a spoon and enjoy.