Coat ten 1-cup ramekins with softened butter, then coat inside of dish evenly with the 3 tablespoons sugar; refrigerate until ready to use.

Melt chocolate and 6 Tablespoons butter in medium bowl set over pan of simmering water. Turn off heat, stir in salt, vanilla, and liqueur; set aside.

Bring the ½ cup of sugar and 3 tablespoons water to boil in small saucepan, then simmer until sugar dissolves.

Beat egg whites until frothy; add cream of tartar and beat to soft peaks; add confectioners’ sugar; continue beating to stiff peaks. (Mixture should just hold the weight of a raw egg in the shell when the egg is placed on top.) Set aside in large bowl.

With mixer running, slowly add sugar syrup to egg yolks; beat until mixture triples in volume, about 3 minutes. Fold into chocolate mixture.

Vigorously stir one-quarter of whipped whites into chocolate mixture. Gently fold remaining whites into mixture until just incorporated. Fill each ramekin almost to rim, wiping excess filling from rim with wet paper towel. Cover and freeze until firm, at least 3 hours and up to 1 month before baking.

Adjust oven rack to lower middle position and heat oven to 400 degrees. Bake until fully risen, about 16 to 18 minutes. (Soufflé is done when fragrant and fully risen. If not yet done, place back in oven.) Serve immediately.