Preheat oven to 350 degrees. Grease a 9 x 13" baking pan. Non-stick or aluminum is best but glass can work. In a bowl, cream together soft butter, brown sugar and egg yolk. Stir in flour and mix well. Spread dough evenly in pan. Bake for 15 minutes or until light golden. Cover the top immediately with chocolate chips. When the chocolate is glossy and melted, spread it evenly with a knife and sprinkle with nuts if desired. Score and cut while warm.