Preheat oven to 350F.

Lightly grease a 9×13-inch baking pan; set aside. In a large mixing bowl, mix together the brown sugar and flour. Using a pastry cutter or 2 knives (I use a fork), cut in the butter until the mixture resembles coarse crumbs. Remove one cup of mixture and set aside.

To mixture in large bowl, add baking powder and salt. Using a fork, lightly beat in vanilla, milk and egg. Continue beating until a smooth batter forms. Pour batter into prepared baking pan.

In a small bowl combine the chocolate chops and walnuts. Sprinkle reserved crumb mixture over top of batter in pan. Sprinkle with the chocolate chops and walnuts. Using a long flat spatula, spread topping evenly over the top of the batter in pan.

Bake bars for 35 minutes, or until a tester inserted in center comes out clean. Cool bars completely before slicing. Use a serrated knife to cut. Store in airtight container.