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 dry ingredients. In a small mixing bowl, whisk together the flour, salt, and baking powder. Set aside.
Combine wet ingredients. In a large mixing bowl, whisk together the granulated sugar, eggs, brown sugar, oil, and vanilla.
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.
Add chocolate mixture to sugar mixture. Whisk the chocolate/butter mixture into the sugar/egg mixture until smooth and combined.
Add dry ingredients. Stir, using a spatula (or wooden spoon) in the dry ingredients until just combined.
Bake brownies. Spread batter into the prepared pan and bake for 22 to 27 minutes until a toothpick inserted in the center comes out with a few moist crumbs clinging to it. Transfer the pan to a wire rack to cool completely before cutting into bars and serving.