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.