Make cupcakes according to box directions. Stir in broken pieces of one Cookies & Cream bar and portion out 1/4 cup of batter into prepared cupcake liners. Bake to box directions.
Make the frosting by melting the second bar in the microwave in 20 second bursts, stirring in between each heating to prevent scorching. Stir in the icing sugar to achieve your desired texture - add in a bit of butter if the frosting gets too thick and stiff.
Frost cupcakes with a Wilton 1A frosting tip and garnish with a small square of Hershey's Cookies & Cream chocolate.