Preheat oven to 350 degrees.

Grease an 8x8 square baking pan or line with parchment and set aside. If lining pan, leave some hanging over the edges to easily pull brownies from the pan once cooled.

In a large mixing bowl, combine melted butter, sugars, and vanilla. Beat in eggs.

In a separate bowl, sift together dry ingredients - flour, cocoa, and salt. Slowly stir dry ingredients into egg mixture until combined.

Stir in chocolate chips (or other mix-ins of your choosing).

Pour into 8x8 pan and use a spatula to spread evenly to the edges. Batter will be thick.

Bake for 20-24 minutes, then test with a toothpick for doneness. Insert toothpick into the center of the pan. If toothpick comes out with wet batter, the brownies are not done. Continue checking every 2 minutes. If toothpick comes out with a few crumbs, then the brownies are done.

Place brownies onto a wire rack and allow to cool completely before frosting.

In a large bowl, cream confectioner's sugar and butter until light and fluffy. Beat in cocoa powder and vanilla. Add milk one tablespoon at a time and continue beating until desired consistency is achieved.

Spread frosting evenly over brownies and cut into bars.