Tint 1 cup buttercream dark red with gel-paste food color, and transfer to a pastry bag fitted with a small plain tip (No. 3). Place white round candy sprinkles (nonpareils) in a small bowl. Using an offset spatula, spread a smooth layer of untinted buttercream over each cupcake.
Dip each frosted cupcake in sprinkles, rolling it around to coat completely. Pipe two red curved lines on each cupcake for the seams, then pipe short lines angled off both sides of curved lines to mimic small stitches. Refrigerate 30 minutes to allow frosting to set.
Once decorated, cupcakes can be stored up to one day at room temperature in airtight containers.