Prepare the cake mix according to box directions. Portion out 1/3 cup of batter into each liner of a lined muffin tray.
When the cupcakes are a bit puffy but not completely baked, about the 12 minute mark, remove from the oven and add a square of chocolate to the top of each cupcake.
Prepare chocolate buttercream frosting by blending the butter, cocoa, icing sugar, and vanilla. Use the cream to adjust desired consistency.