Preheat oven to 350°F. Prepare are 9x13 baking pan by lining it with parchment paper. I like to have 2 of the ends hanging out so it's easy to pull the brownies out.
In a microwave safe bowl, melt together the butter and chocolate. Melt in 20-30 second increments, stirring after each time. Set aside so it can begin to cool.
Using a hand or stand mixer, beat together the eggs and sugars until light and fluffy (about 2 minutes). Once your chocolate has cooled, add that to your egg & sugar mixture. Add the vanilla. Mix until combined.
Using a spatula, gently fold in the the dry ingredients until everything is combined. Be careful not to over mix.
Pour the batter in the prepared pan and bake for 30-32 minutes. Your brownies are done when the sides start to pull away from the pan, the center is not jiggly, and you have moist crumbs on your toothpick. Remember, brownies continue to bake in the pan after you pull them out. We don't want to over bake these!
Let the brownies cool. I like to put my pan on a wire rack to help this process. While the brownies cool, you can begin to make the mint frosting layer.