Combine first 3 ingredients in a microwave-safe bowl. Microwave at HIGH for 45 seconds or until chocolate melts, stirring after 20 seconds; stir until smooth. Stir in liqueur.
Place egg whites in a large bowl; beat with a mixer at medium-high speed until soft peaks form. Combine 1/4 cup sugar, 2 tablespoons water, and salt in a small saucepan; bring to a boil. Cook, without stirring, until candy thermometer registers 240°. Gradually pour hot sugar syrup in a thin stream over egg whites, beating at medium speed. Increase speed to high, beating until stiff peaks form.
Fold half of egg white mixture into chocolate mixture. Fold in remaining egg white mixture. Gently fold in whipped topping. Divide evenly among 6 bowls. Cover surface of mousse with plastic wrap; chill at least 2 hours.
Place wafers 2 inches apart on parchment paper, flat side down. Top each with 1 teaspoon nuts. Place remaining 1/4 cup sugar in a clean, small, heavy saucepan; cook over medium heat until sugar dissolves and is golden (about 3 minutes). Do not stir. Drizzle sugar evenly over wafers. Let stand for 5 minutes or until set. Remove plastic wrap from dishes; top each serving with 1 wafer.
Nutrition Note: If you substitute an equivalent amount of whipped cream for the whipped topping in this recipe, you will add 41 calories, 8g fat, and 2g sat fat to each serving.