Preheat the oven to 375 degrees F (190 degrees C). Grease a 9x13 inch baking pan. In a heavy saucepan combine the butter and unsweetened chocolate. Cook over low heat, stirring constantly until smooth and well blended. Remove from heat and set aside.
In a large bowl, beat eggs, sugar and vanilla until thick and pale. Stir in the chocolate mixture and coffee liqueur. Fold in the flour. Stir in chopped walnuts if desired. Spread evenly into the prepared pan.
Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out almost clean. Be careful not to overbake. Cool for at least 30 minutes before cutting into bars and serving.
