1. Whisk sugar and cornstarch in a medium saucepan to blend. Gradually whisk in milk, then egg yolks. Bring to a boil over medium heat, stirring occasionally. Reduce heat to low and, stirring gently, cook 2 minutes to cook yolks. Remove from heat; stir in vanilla. Add chocolate; let stand 1 minute to melt, then whisk until blended. Refrigerate 15 minutes until slightly cooled.
2. Spread 1/3 cup chocolate custard over bottom of crust. Peel and thinly slice bananas; arrange over custard. Spread with remaining custard. Place plastic wrap directly on surface and refrigerate at least 5 hours or up to 1 day.
3. Up to 3 hours before serving: Beat Topping ingredients in a medium bowl on high speed until stiff peaks form when beaters are lifted. Sprinkle 1/2 cup toffee bits over custard. Drop dollops of topping on pie, or scrape topping into a ziptop bag, cut 1/2 in. off tip of 1 corner and pipe a design on pie. To garnish, sprinkle with additional toffee bits.