In the microwave, very carefully melt chocolate (do NOT overheat) & add butter. Stir until combined & slightly cool; add beaten egg & sugar. In a small bowl, combine flour, baking powder & salt. Add to chocolate mixture alternately with combined milk & vanilla. Fold in walnuts. Carefully spread batter over bottom layer. Bake for 20-25 minutes; do not OVER BAKE. Remove from oven & cool. Slice into 20 squares.