Preheat the oven to 350° F. Butter and flour a 9x13-inch baking pan.
In a medium bowl, whisk the flour, baking powder, and salt until well combined. Place the butter and chocolate in a microwave safe bowl. Microwave in 30 second increments, stirring each time, until melted. Transfer to a large bowl. Add the brown sugar into the melted chocolate with a whisk or an electric mixer at medium speed until the mixture is smooth and silky. Beat in the eggs and vanilla until just incorporated.
With a wooden spoon, stir in the flour mixture and any add-ins desired until just combined. Do not beat. Spoon the batter into the prepared pan, spreading it gently into the corners.
Bake for 35 minutes, or until a toothpick comes out with a few moist crumbs attached. Cool the pan on a wire rack for 30 minutes (yeah right!). Cut the brownies into 24 pieces while they’re still in the pan, and remove with an offset spatula.
Notes
Cook’s notes: *I would try using half cake and half all-purpose. Next time I bake these I’ll post what I think about the combination, but I would guess that you’ll get a lighter brownie. If you want to stick with the tried and true, go with all “all-purpose” for now. **I like using a large serrated bread knife to chop chocolate bars. It makes easier work of the chopping. I also have one of those chocolate chippers, but I think it makes a big mess.