1. Preheat the oven to 350 degrees. Grease and flour a 9x13-inch baking pan. Set aside.
2. In the bowl of a stand mixer, combine the butter and brown sugar. Beat on medium speed with the paddle attachment until it's uniformly brown and grainy. Add the eggs and vanilla and beat well on medium speed to combine (about 1 minute).
3. Meanwhile, whisk together the flour, baking powder and salt. With the stand mixer running on low, add a little at a time to the butter mixture until just incorporated. Add all the chocolate and mix briefly to combine.
4. Pour the batter into the prepared pan and spread out into one even layer as best you can -- don't worry if it doesn't quite reach end to end though. The batter will spread while cooking.
5. Bake for 23-27 minutes, until golden. Let cool for 20 minutes before cutting into 12 bars. Enjoy!