Using a sharp paring knife, cut a 3-inch-wide slit into one side of each bread slice, cutting about three-quarters of the way through to make a pocket.
Spread 1/2 tablespoon of the chocolate-hazelnut spread inside each pocket and then layer in 3 to 4 banana slices.
Heat a large skillet over medium heat and add enough butter to liberally coat the bottom. Heat until hot, but not smoking.
Meanwhile, whisk together the cream, eggs, sugar and cinnamon in a medium bowl. Soak each stuffed bread slice into the custard and, working in batches, fry in a single layer until the custard is fully cooked, about 3 minutes per side.
Top the French toast with additional banana slices and serve warm with a drizzle of maple syrup.