Start by piping a thin, even layer of buttercream on top of a cupcake
Use a hot spatula to smooth out the top.
Roll the sides of the cupcakes in small sprinkles.
Place a small cookie cutter on top of the frosted cupcake.
Pour mini sprinkles inside the cutter, and press down gently.
Remove the cutter from the top of the cupcake. Voila!