These bars are an annual tradition in my family. The kids love to help especially with the chocolate. I love dark chocolate while others in the family love milk chocolate so we usually end up making several batches before Christmas even gets here.
Cream butter and sugar together. Add egg, flour and vanilla and beat until well incorporated.
Spread in the bottom of an ungreased jelly roll pan (11x17). Batter will be thick so spread it out evenly. Bake at 350 degrees for 15 minutes. Remove from oven and top with the broken chocolate pieces and return to the oven for 1-2 minutes. Then spread chocolate evenly. Cut when cool.