Sift flour, measure, then sift again with baking powder and salt. Melt butter in saucepan-remove from heat and stir in sugar. Cool slightly, then beat in egg thoroughly. Add vanilla. Fold in flour mixture. Stir in apples and nuts. Spread in 8x8 pan. Bake at 350 degrees 30 minutes. Cut in 2x2 squares while still warm. Makes 16 cookies.