Preheat oven to 350°F. Butter a 9-inch deep-sided springform pan, line with parchment paper, and butter the parchment.

In a large saucepan, simmer chocolate, unsalted butter, and liqueur. When mixture is melted and smooth, stir in vanilla extract. Set aside to cool.

Beat eggs with granulated sugar on medium for 5 minutes or until fluffy. Gradually add eggs to chocolate mixture. Spoon batter into pan.

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.

When cool, sprinkle cake with confectioners' sugar. Store in refrigerator.