Make cake. In small bowl, mix multicolor jimmies, nonpareils and jumbo nonpareils together from 6-cell sprinkles mix. Set aside.

Prepare cake batter following recipe instructions. Stir 3 tablespoons of sprinkles mix into batter, reserving remaining sprinkles for later step.

Bake and cool two cake layers.

Ice cake. Prepare two batches buttercream icing following recipe instructions.

Level cake layers to 2½ in. high. Fill and stack layers for 5 in. high cake.

Ice cake smooth.

Decorate cake. Press reserved sprinkles around sides of cake, heavier at the bottom then lighter towards the top.

Prepare decorating bag with tip 32 and remaining icing. Pipe swirls around top edge of cake. Insert one icing decoration into each swirl. Insert candles, if desired.