Place chocolate in top of a double boiler over simmering water, and cook over low, stirring occasionally, until chocolate is melted. Remove from heat, and set aside.

Place egg yolks and 3 tablespoons water in a heavy saucepan; cook over very low, whisking vigorously and constantly, until yolks begin to foam and thicken, about 6 minutes. Whisk in liqueur, and cook, whisking constantly, until sauce thickens enough to coat the back of a spoon, 6 to 8 minutes. (The sauce should achieve the consistency of a hollandaise or sabayon.) Remove from heat. Pour through a fine wire-mesh strainer into a bowl; discard solids.

Fold melted chocolate into sauce. Transfer chocolate mixture to a large bowl, and set aside.

Beat cream with an electric mixer on high speed until stiff peaks form, 2 to 3 minutes, beating in 2 tablespoons sugar toward the end. Fold into chocolate mixture.

Using electric mixer fitted with the whisk attachment, beat egg whites on high speed until soft peaks start to form, about 1 minute. Beat in remaining 1/4 cup sugar and continue beating until stiff peaks form, 2 to 3 minutes. Fold into chocolate-cream mixture.

Spoon mousse into a bowl, and chill until ready to serve, 4 hours or up to 24 hours. Garnish servings with whipped cream and grated chocolate.