1.Preheat the oven to 350°F. Line a baking sheet with parchment paper. Have a small glass of water ready. In a small bowl, mix the sugar and cinnamon with a fork until combined well.
2.Lay 4 wonton wrappers out on a clean countertop. Place 1 teaspoon Nutella in the center of each wrapper. Cut the banana into very thin slices with a paring knife and place 2 slices on top of the Nutella on each wrapper. Dip your finger into the glass of water and run it along the edges of a wonton wrapper. Fold the wrapper in half diagonally, forming a triangle. Press on the edges well with your fingers to seal. Repeat with the remaining 3 wontons. Place the 4 on the baking sheet (you can place them close together; they won’t spread).
3.Repeat with the remaining wonton wrappers, Nutella, and bananas, working in batches of 4 wontons at a time. When all the wontons have been made, lightly brush the sides facing up with the melted butter and sprinkle with the cinnamon-sugar. Place the baking sheet in the oven and bake the wontons for 8 to 10 minutes, until crisp and lightly browned. Let cool for 1 minute before serving.