Pre-heat oven to 350 degrees
Combine flour and baking soda in small bowl.
Beat brown sugar, peanut butter, butter and sugar in large mixer bowl until creamy.
Beat in egg and vanilla.
Gradually beat in flour mixture, stir in chunks.
Press into ungreased 9x13 inch baking pan.
Bake 18-22 minutes or until center is set.
Cool completely in pan on wire rack.
Cut into squares.
Then melt some chocolate chips with some peanut butter and drizzle over top of squares.