In a large mixing bowl, beat butter on high speed for 30 seconds. Add the brown sugar, egg yolk, and vanilla. Beat until combined, scraping sides of bowl occasionally. Beat in flour and salt until combined.
Spread dough evenly in an ungreased 13x9inch pan. Bake for about 25-30 minutes or until edges begin to brown and surface is dry. Remove from oven and immediately sprinkle with chocolate pieces. Let stand about 2 minutes or until chocolate is softened; spread evenly.
Cool in pan on a wire rack about 30 minutes. Cut into bars and serve.
Notes
I usually use semi-sweet chocolate chips, but milk or dark also work well.Recipe from my Betty Crocker cookbook.