It's two favorite desserts in one--a shortbread crust with a chocolate brownie on top.
Preheat oven to 350 degree F. Line a 9x9x2-inch baking pan with foil; set aside. In a medium mixing bowl stir together 1 cup flour and the brown sugar. Cut in the 1/2 cup butter until mixture resembles coarse crumbs. Stir in the 1/4 cup chocolate pieces. Press into prepared pan. Bake for 8 minutes.
Meanwhile, in a large mixing bowl stir together granulated sugar, 3/4 cup flour, the cocoa powder, baking powder, and salt. Add eggs, melted butter, and vanilla; beat by hand until smooth. Stir in the 1/2 cup chocolate pieces. Carefully spread over crust in pan.
Bake 40 minutes more. Cool brownies in pan on a wire rack. Lift brownies out of pan by lifting up on foil. Cut into bars. Makes 24 bars.