Build charcoal fire or preheat gas grill.
Slice bananas in 1/2 lengthwise and crosswise so each banana yields 4 pieces. Set aside. Mix the sugar and cinnamon, sprinkle on cut sides of bananas. Let bananas sit for 5 minutes.
Place bananas cut-side down on a clean cooking grate over direct low heat. Cook for 2 minutes or until grill marks appear. Using a pair of long-handled tongs, turn over and let cook 5 more minutes or until the skin pulls away from the banana.
Remove skins from banana halves and place 4 banana halves in the bottom of each serving bowl. Top each serving with 2 scoops of ice cream. Ladle about 1/4 cup of heated chocolate sauce over each sundae¿let your taste determine the quantity here, add just a touch for a hint of chocolate or pour it on for a serious chocolate fix. Sprinkle each with 1 tablespoon of chopped nuts, top with whipped cream, and place a cherry or strawberry on top.
Make the chocolate sauce up to 2 days in advance. Place chocolate in a medium size metal mixing bowl. Heat cream to almost boiling, add butter and sugar and stir to combine. Remove from heat and pour over chocolate. Whisk vigorously until melted, and well combined. Add liqueur and vanilla and stir constantly until mixture is cool the touch. Cover with plastic wrap and set aside or refrigerate up to 2 days. Warm before serving.
Recipe courtesy Elizabeth Karmel, Girls at the Grill