Butter bottom and sides of 10 (6-ounce) custard cups or soufflé dishes, and sprinkle lightly with sugar. Set aside.
Melt 3 tablespoons butter in a heavy saucepan over low heat; whisk in flour. Cook, whisking constantly, 1 minute. Gradually whisking milk; cook over medium heat, whisking constantly, until thickened and bubbly. Whisk in 2/3 cup sugar and salt.
Remove from heat. Add chocolate and liqueur, whisking until chocolate melts.
Beat egg yolks until thick and pale. Gradually stir about one-fourth of chocolate mixture into yolks; add to remaining chocolate mixture, stirring constantly. Stir in vanilla extract.
Beat egg whites at high speed with an electric mixer until stiff peaks form. Fold into chocolate mixture. Spoon into prepared custard cups. Place cups in a 15- x 10-inch jellyroll pan.
Bake at 350° for 30 to 35 minutes or until tops are puffed and appear done. Invert onto individual serving plates, and pipe with White Chocolate Mousse; serve with Mocha Sauce. Garnish, if desired.
Note: To chill soufflés, invert into serving plate; cover and chill. White Cholcolate Mousse and Mocha Sauce may be omitted and soufflés topped with whipping cream.