Prep oven and pan. Position a rack in the center of your oven and preheat the oven to 350°C (180°C). Line an 9"x13” pan with parchment paper, or grease with non-stick cooking spray. See note below about using a metal or glass pan.
Combine butter and chocolate. In a small saucepan on the stove, over low heat, melt and stir together the butter and chopped chocolate. Stir frequently to keep the chocolate from burning. Remove from the heat and stir in the cocoa powder, whisking to combine. Allow to cool slightly.
Combine wet ingredients. In a large mixing bowl, whisk together the granulated sugar, eggs, brown sugar, oil, and vanilla.
Add chocolate mixture to sugar mixture. Whisk the chocolate/butter mixture into the sugar/egg mixture until smooth and combined.
Add dry ingredients. Stir in the flour, salt, and baking powder.
Spread batter. Spread the brownie batter into an even layer into the bottom of the prepared pan with a rubber spatula.