In a large mixing bowl, mix together the brown sugar and the flour, using a pastry cutter or two knifes, cut in the butter until mixture resembles coarse crumbs. Remove 1 cup of this mixture and set aside.
To mixture in large bowl, add the baking powder and salt. Using a fork lightly beat in vanilla, milk and egg. Continue beating until smooth. Pour batter into a 13x9 greased pan.
In a small bowl, combine the chocolate chips and walnuts.
Sprinkle reserved crumb mixture over top of batter in pan. Sprinkle with the chocolate chips and the walnuts. Using a long flat spatula, spread topping evenly over the top of the batter in pan.
Bake bars for 35 - 45 minutes, or until a skewer inserted in center comes out clean. Cool bars completely before slicing. Cut into bars, should make about 24.