In a large mixing bowl combine brownie mix, water, oil, and egg. Stir until well combined. Spread dough evenly into a greased 12-inch pizza pan with 1/2-inch sides. Bake in a 375 degree F oven for 18 to 20 minutes. Cool completely.
Meanwhile, for the White Chocolate Sauce, in a heavy saucepan bring whipping cream and vanilla just to boiling, stirring frequently. Remove saucepan from heat. In a small mixing bowl beat 1 egg yolk, sugar, and 2 to 4 tablespoons of the hot whipping cream mixture with an electric mixer on medium speed for 2 to 3 minutes or until thick and lemon-colored. Gradually stir about half of the remaining whipping cream mixture into the egg yolk mixture. Return all to the saucepan. Cook and stir over medium heat just until mixture returns to boiling. Remove from heat. Stir in grated white baking bar. Continue stirring until bar melts. Cover surface with plastic wrap; cool for 15 minutes. Stir before serving. Store any remaining sauce in the refrigerator.
Cut crust into 12 wedges; do not remove from pan. Place small scoops of chocolate ice cream on each wedge. Cover tightly and freeze until firm.
Just before serving, pour White Chocolate Sauce over ice cream. Sprinkle with cut-up candy bars. If desired, sprinkle with chopped nuts and garnish with fresh strawberries. Serve immediately. Makes 12 servings.