Melt butter or shortening over low heat. Stir in brown sugar. Remove from heat. Sift together and stir in flour, baking powder, vanilla, salt and walnuts. Spread in well-greased and floured square 8 x 8 x 2-inch pan. Bake 20 or 25 minutes in moderate oven (350°). Appears soft when removed from oven because interior is supposed to be soft. Cut while warm.