-(do not substitute) Cake: Place oven rack in lower third of the oven, and preheat oven to 375 degrees. Butter an 20 cm springform pan, and coat with powdered cocoa. Melt chocolate and butter in a double boiler over hot, but not boiling, water. (don't rush this step, chocolate burns easily) Beat egg yolks in a lg mixer bowl at high speed, gradually adding 0.75 c of the granulated sugar. Beat until yolk mixture is pale yellow and thick, 4 to 6 minutes. Add melted chocolate mixture to the yolk mixture. Beat until completely smooth. Add vanilla and liquor of your choice. Beat until blended. Beat egg whites in mdm mixer bowl at high speed until soft peaks form. Gradually beat remaining quarter c granulated sugar into whites; continue beating until stiff but not dry peaks form. Fold whites gently but thoroughly into chocolate mixture. Pour batter evenly into pan; smooth top. Bake 15 minutes. Reduce oven to 350 degrees; bake another 15 minutes. Reduce oven temp to 250; bake 30 minutes longer (total baking time is 1 hour). Turn off oven; prop open oven door and allow cake to remain in oven for 30 minutes. Remove cake from oven and cover top with damp paper towelling; let stand 5 minutes. Remove towelling and cool cake completely. Dome of cake will crack and collapse; this is normal press top of cake down lightly to smooth top. Remove springform and transfer cake to serving platter. To make the frosting: Melt chocolate in double boiler over hot but not boiling water. Using a wooden spoon, mix in sour cream. Pour over the cake. Let cool and harden.