Preheat oven to 350°. Prepare 9 x 13 pan with pan spray or shortening, line with parchment paper.

In large mixing bowl, combine sugars, flour, cocoa, baking powder, and salt. Set aside.

In a smaller bowl, whisk together melted butter, eggs, and vanilla extract.

Add egg mixture to the flour mixture, stir until dry ingredients are well incorporated.

Spread batter into prepared pan, Bake for 30 min. Brownies are done when knife inserted into center comes out just barely clean - but don’t OVER cook them.

Let cool completely before proceeding.

Whip butter until smooth. Add Vanilla and salt, mix until well incorporated.

Slowly add the powdered sugar a little bit at a time, until incorporated completely.

Beat on high for 1 minute - mixture will be very, very thick and probably crumbly. Lower mixer speed to lowest setting, and slowly add ½ of the milk. Beat until smooth.

Add remaining powdered sugar and remaining milk - alternating - and continue to beat until smooth.

Once incorporated, check for consistency. Add more milk or sugar to achieve the consistency you want or need.

Fit a large piping bag with a coupler and a flat tip, fill with frosting. Make sure to avoid air pockets as much as possible.

Use the parchment paper to carefully lift the slab of brownie out of the pan. Be careful to support it and not allow it to bend / break as your transfer it to a flat cutting surface.