Preheat oven to 350°F. Butter 9" springform pan and line it with parchment paper. Butter the parchment.

In a large saucepan, simmer the chocolate, butter and liqueur until melted and smooth. Stir in the vanilla extract. Remove from heat and allow to cool. Beat the eggs with the sugar on medium speed until fluffy, about 5 minutes.

Gradually add eggs to chocolate mixture. Pour into pan and bake for about 1 hour. Tester should come out with a few crumbs attached.

Sprinkle confectioners sugar, cocoa, or any combination before serving.

Store in refrigerator.