Preheat your oven to 350 degrees. Generously grease your brownie pan- you can use an 11x7 inch brownie pan, or an 8x8 inch square cake pan.
In a large bowl, whisk together the butter, oil, sugar, and brown sugar until well combined. Add in all 3 eggs and the vanilla extract, and whisk together until well combined. In a small bowl, stir together the flour, cocoa powder, salt, and baking powder, and then stir this mixture into your large bowl. At this point you want to whisk the batter for a full 5 minutes by hand. It's going to seem like a really long time to mix this batter, but trust me! It helps create the perfect texture.
Once you've mixed the batter well, switch to a rubber spatula and fold in the chopped chocolate. Pour the batter into your prepared pan, and spread it evenly. Bake the brownies for 22-25 minutes if you are using an 11x7 inch pan, or 32-35 minutes if you are using an 8x8 inch pan. The brownies should have a shiny top, set edges, and toothpick inserted about 2 inches from the edge of the pan should come out with a few crumbs on it. Cool the brownies, slice, and enjoy!