Line the bottom of a 9 X 13 pan with graham crackers. Mix butter, brown sugar and pecans in a saucepan. Boil for two minutes and then pour over graham crackers. Bake at 350° for 9 minutes. After removing from oven, lay the chocolate bars, evenly spaced, in pan. When the chocolate is melted (2-3 minutes), spread over entire surface. Cool, cut into pieces.