Melt butter in medium saucepan over medium heat. Continue cooking until butter stops foaming and there are brown bits on the bottom of the pan.

Transfer butter to a bowl, add in brown sugar and whisk until well combined. Cool for 5 minutes. Whisk in eggs, then add vanilla and stir to combine.

Whisk together flour, baking powder, and salt. Pour into the batter mix, then whisk. Fold in chopped chocolate, then turn batter into prepared pan.

Bake for 26-28 minutes, or until top is light golden brown. Cool in pan on wire rack, then transfer from pan and cut into 2×2 squares.