Preheat oven to 350.
Line a 13X 9 inch pan with foil, with ends extending over the sides of the pan. Grease foil.
Melt chocolate and butter over low heat in a sauce pan and stir in sugar. Mix well and stir in the eggs and vanilla. Add flour and mix well. Add any additions at this time. Spread into prepared pan and bake 30 to 325 minutes until a wooden toothpick inserted in the center comes out with fudge crumbs.
Cool. Remove from the pan by using foil handles. Cut into squares.
Store in tightly covered container at room temperature.