For Crust: In a large saucepan melt 1 cup butter and unsweetened chocolate over low heat. Remove from heat. Stir in sugar and vanilla. Then add eggs and beat just until combined. Stir in flour. Spread chocolate mixture in a greased 15 by 10 inch baking pan. Bake at 350 degrees for 25 minutes. Remove from oven and cool. Crust should be moist.
For Filling: In a large mixing bowl place powdered sugar and the 1/2 cup softened butter. Beat with mixer until combined. Gradually add half and half and creme de cacao. Beat well. Stir in nuts, if used. Spread filling over cooled crust. Chill until filling is firm.
For topping: In a small saucepan on low heat, melt chocolate and butter. Remove from heat and add creme de cacao. Stir until smooth and then drizzle over filling. Chill slightly and then score bars where you will cut them. This prevents the pretty drizzle from just cracking and looking broken when cutting to serve. Then chill bars until firm. These will keep in refrigerator for up to 3 days.