Melt butter and chocolate in a saucepan on medium heat. Stir constantly until completed melted and combined.
Remove from heat and pour into mixing bowl, allow to cool for about 10 minutes.
Preheat oven to 350 degrees. Line the bottom of a 9x13 baking pan with foil or paper and leave an overhang on the sides. (set aside)
Whisk granulated sugar and brown sugars into the cooled chocolate/butter mixture. Add eggs one at a time and whisk until smooth. Whisk in the vanilla.
Gently fold in salt, flour and cocoa powder. Pour into your baking pan and bake for 35 minutes. My oven runs hot, so it took only 30 minutes to cook completely. It's highly recommended to check your brownies at 30 minutes. (You know its complete when you insert a toothpick and only a few crumbs come out.)
Brownies need to completely cool IN THE PAN before you move them to a wire rack. Once cool, lift the foil and move to the wire rack. Leave uncut and to the side until you have the mint frosting layer ready.