Cupcakes:

 Preheat oven to 350°.

 Measure out brownie ingredients (excluding the buttercream ingredients). Put the eggs directly into your mixer bowl, and mix on low speed just until incorporated. Beat on high speed for 2 minutes. 

Add eggs, and then beat on high speed again for about 2 minutes.

 Transfer batter into a container with a pour spout and pour the batter into ice cream cones, leaving a good ½ inch between the batter and the top of the cone.

 Bake at 350° for about 20 minutes or until the cake tester comes out clean.

Buttercream:

Beat butter until creamy, and then scrape the bowl.

 Add 5 cups  sifted powdered sugar, milk, and vanilla, and then beat until combined.

 Add more powdered sugar until you get the right consistency (not too stiff so that its hard to pipe).

 Take out half of the butter cream and reserve. This is the vanilla.

 Add cocoa powder to the remaining half and beat to combine, you might need to add a little milk if its too stiff. This is the chocolate butter cream. 

Pipe frosting onto cooled cupcakes with a large star tip. Top with sprinkles.