HEAT oven to 300 degrees F. Whisk eggs in large shallow bowl. Whisk in half-and-half, sugar, cinnamon and vanilla. Coat electric griddle or large nonstick skillet with cooking spray. Heat griddle to 350 degrees F or skillet over medium heat. Dip bread slices in egg mixture, about 5 seconds each, turning to coat both sides. Cook on prepared griddle until both sides are golden brown. Place on baking sheet while cooking remaining French toast.
SPREAD one side of each of 4 slices cooked French toast with a rounded tablespoon of chocolate hazelnut spread. Cut 2 bananas in half lengthwise, then in half crosswise. Melt butter in medium nonstick skillet over medium heat. Add banana. Sprinkle with brown sugar. Cook 3 to 4 minutes or until golden brown, turning halfway through cooking time. Remove from skillet, reserving sauce. Arrange banana pieces evenly over spread on French toast. Top with remaining bread slices to form 4 sandwiches.
COVER sandwiches with foil. Bake 8 to 10 minutes to heat through. Slice remaining banana, adding to sauce in skillet. Saute 2 minutes or until golden brown. Stir in syrup. To serve, cut French toast on a diagonal; arrange on serving plates. Sprinkle with powdered sugar and/or cocoa powder. Top with banana-syrup mixture. Garnish with whipped cream, if desired.