Grease a 8x8 inch baking pan. Set aside.
Preheat oven to 350.
In a medium bowl mix flour, baking powder, and salt.
In a double boiler, melt the butter and the chocolate.
Once melted stir to incorporate.
Remove from heat, add brown sugar, and granulated sugar.
Mix, then add eggs, and yolk and vanilla.
Mix well.
Add flour mixture and FOLD together until well incorporated. Do not over mix.
Fold in chocolate chips.
Pour into prepared pan and bake for 25-30 minutes until skewer in center comes out clean.
Cool completely and then frost.