Prepare the Crepe Batter: In a large mixing bowl, combine the all-purpose flour, eggs, milk, water, vanilla extract, and sugar. Whisk until the mixture is completely smooth and free of lumps. Let the batter rest at room temperature for about 30 minutes.
Cook the Crepes: Preheat a non-stick skillet over medium heat and lightly grease it with melted butter. For each crepe, pour approximately ¼ cup of the batter into the skillet, swirling it around to coat the bottom evenly. Cook for 1-2 minutes until the edges begin to lift. Flip the crepe and cook for an additional minute until lightly golden. Remove and set aside. Continue with the remaining batter.
Assemble the Cake: On a serving plate, lay down the first crepe. Spread a thin layer of Nutella over the top. Arrange banana slices on top of the Nutella. Place another crepe on top and repeat the layering process until all crepes are stacked, finishing with a plain crepe on the top.
Chill the Cake: Cover the assembled cake tightly with plastic wrap and refrigerate for at least 2 hours.
Serve: Remove the cake from the refrigerator and slice into wedges. Top each slice with whipped cream and chopped hazelnuts if desired.
Notes
For an extra indulgent touch, drizzle additional Nutella over the top before serving.