In small mixer bowl, combine sweetener, butter, egg and vanilla. Beat 1 1/2 minutes at high speed. Measure flour by lightly spooning into cup and leveling off. Add remaining ingredients except chocolate pieces and nuts. Blend well; beat 2 minutes at medium speed, scraping sides of bowl. Stir in chocolate pieces and nuts.
Pour batter into an ungreased 8 inch square baking pan. Bake at 375 degrees for 20-22 minutes. Cool. Cut into bars.
Makes 24 bars.