Preheat oven to 350 F. Lightly grease a 13x9-inch baking pan; set aside. In a large mixing bow, mix together the brown sugar and flour. Using a pastry cutter or two knives, cut in the butter until mixture resembles coarse crumbs. Remove 1 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 chips and walnuts. Fold in the coconut.
Sprinkle reserved crumb mixture over top of batter in pan. Sprinkle reserved crumb mixture over top of batter in pan. Sprinkle with the chocolate chips 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 skewer inserted in center comes out clean. Transfer pan to a wire rack. Cool bars in pan completely before slicing. Using a serrated knife, cut into 24 bars. Store in airtight container for up to 5 days.