Preheat oven to 325°. Grease and line with parchment paper one 8x8-inch cake pan.
Place the chocolate and butter in a large heat-proof bowl. Melt over a double boiler. Stir occasionally until smooth. Carefully remove from heat.
Add both sugars to the chocolate mixture and whisk until smooth and well combined. Add eggs and vanilla, whisk the mixture for about 1-2 minutes until well combined. Add flour, salt, and baking powder. Stir mixture until smooth.
Pour batter into the prepared pan and bake for 40-45 minutes. Remove from oven and allow the brownie to cool in the pan for 15 minutes. Remove from pan and cool completely. Cut into 16 pieces.