Place egg yolks in a medium bowl. Combine 3 tablespoons water, butter, and chocolate in a large, heavy saucepan over medium heat, stirring constantly until chocolate melts. Gradually add chocolate mixture to egg yolks, stirring constantly with a whisk. Return chocolate mixture to pan; cook over medium heat until thick (about 3 minutes), stirring constantly. Remove from heat; stir in liqueur. Cool slightly.
Place cream of tartar, salt, and egg whites in a large bowl; beat with a mixer at high speed until foamy. Combine sugar and 2/3 cup water in a saucepan; bring to a boil. Cook, without stirring, until candy thermometer registers 238°. Gradually pour hot sugar syrup in a thin stream into egg white mixture, beating at high speed until stiff peaks form (5 to 7 minutes).
Gently stir one-fourth of egg white mixture into chocolate mixture; gently fold in remaining egg white mixture. Fold in whipped topping and hazelnuts. Spoon mixture into a chilled freezer-safe container; freeze 8 hours or overnight.