Melt chocolate and butter together. When cool, add liqueur or coffee. Separate eggs. Beat yolks and sugar for 10 minutes. Beat whites until stiff. Fold 1/3 yolks into chocolate; then 1/3 whites; repeat until all are incorporated. Pour into greased and floured 9-10 inch springform pan. Bake at 350 for 25-30 minutes. Refrigerate for at least 4 hours. Serve with whipped cream or powdered sugar.