Sift together flour,baking powder and salt. Set aside
Cream butter,shortening,brown sugar and vanilla. Beat in egg yolk.
Mix in dry ingredient with a spatula. Spread over bottom of lightly greased jelly roll pan.
Beat egg white until frothy. Spread over dough. Sprinkle one half with walnuts, the other with the pecans, or mix if you like.Bake in a 350 degree oven for 20 to 25 minutes or until lightly browned.Cool slightly on wire rack. Cut into bars with sharp knife.remove to wire rack to finish cooling. STORE IN TIGHTLY COVERED CONTAINER