1. Preheat oven to 350°F. Butter a 9-inch deep-sided springform pan, line with parchment paper, and butter the parchment.
2. In a large saucepan, simmer chocolate, unsalted butter, and liqueur. When mixture is melted and smooth, stir in vanilla extract. Set aside to cool.
3. Beat eggs with granulated sugar on medium for 5 minutes or until fluffy. Gradually add eggs to chocolate mixture. Spoon batter into pan.
4. Bake cake for about 1 hour or until a toothpick inserted into center comes out with a few crumbs attached. Cool for 1 hour in pan, then loosen springform and remove.
5. When cool, sprinkle cake with confectioners' sugar. Store in refrigerator.