1. Preheat oven to 300 degrees. Butter the bottom and sides of a 9-inch springform pan. Line bottom with a round of parchment paper; butter parchment. Dust paper and sides of pan with cocoa powder, tapping out excess.

2. In a heatproof bowl set over (not in) a pan of simmering water, melt chocolate and butter, stirring occasionally. Remove from heat; stir in liqueur. In a large bowl, using an electric mixer on high speed, beat egg yolks with 2 tablespoons granulated sugar until pale and doubled in volume, about 6 minutes. Stir in chocolate mixture.

3. In another bowl, with mixer on high speed, beat egg whites until soft peaks form; gradually add remaining granulated sugar until medium-stiff peaks form. Whisk half the whites into chocolate mixture, then gently fold in remaining half.

4. Pour batter into prepared pan; bake until cake pulls away from sides of pan and center is just set, 45 to 50 minutes. On a wire rack, cool completely in pan. Cover with plastic wrap, and refrigerate at least 4 hours or up to overnight. To serve, run a metal spatula around edge of pan, and unmold cake; dust with confectioners' sugar.