Combine the flour, eggs, milk, water, melted butter, sugar, and salt in a blender. Blend until completely smooth, about 30 seconds. Let the batter rest at room temperature for 15 minutes. This rest allows the gluten to relax and the air bubbles to settle, giving you thin, tender crepes.
Heat an 8 or 9-inch nonstick skillet over medium heat. Lightly grease with a small amount of butter.
Pour about 3 tablespoons of batter into the center of the pan and immediately swirl the pan in a circular motion to spread the batter into a thin, even circle. Cook for about 1 to 1.5 minutes until the bottom is lightly golden and the edges start to lift.Flip the crepe with a spatula and cook for another 30 seconds. Slide onto a plate. Repeat with the remaining batter, stacking the crepes with nothing between them.
To assemble, spread about 1 tablespoon of nutella across the center of each warm crepe. Arrange sliced bananas over the nutella in a single line.
Fold the crepes in half over the filling, then fold in half again to form a triangle, or roll them up tightly. Arrange on a serving plate, dust generously with powdered sugar, and serve with whipped cream on the side.