Preheat the oven to 350 degrees. Line a cupcake tin with paper liners and set it aside. Spray gently with non stick spray so they don't stick.
In a large bowl, whisk together the brownie mix, melted butter, eggs, milk and baking powder until well combined.
Use a 2 Tbsp cookie scoop and fill each liner about 3/4 full, should be one scoop per liner. Place the tray in the oven and bake for 20-23 minutes, or until a toothpick comes out cleanly. Remove and let the cupcakes cool completely.
To make the frosting, place the butter in a large bowl and beat with an electric mixer for 1-2 minutes, until the butter lightens in color. Add in the powdered sugar, vanilla and cocoa powder and continue to beat until the frosting is smooth and creamy. Place into a piping bag with frosting tip of choice (#1B shown) and pipe frosting on the top of the cooled cupcakes.
Notes
Brownie Mix Cupcake Tips
Always wait until baked goods are completely cooled before frosting.
You can use a pint size freezer bag instead of piping bag + tip. Just fill, zip closed and snip the corner off to squeeze out.
You could use a tub of store bought chocolate frosting instead this same way to decorate.