Melt the butter in a pan or microwave.
Make the crêpe batter: In a large mixing bowl, whisk together the flour and the egg. Gradually add in the milk and water, stirring to combine. Add the salt and butter; beat until smooth.
Heat a griddle or frying pan over medium-high heat. While the pan is heating, prep the banana.
Cook the crêpes: Lightly grease the griddle or frying pan with nonstick spray. Pour or scoop the batter onto the pan, using approximately 1/4 cup for each crêpe. Tilt the pan with a circular motion so that the batter coats the surface evenly. Cook the crêpe until the bottom is light golden brown, for about 2 minutes. Loosen with a spatula, flip over, and cook the other side until nicely golden brown. Repeat for remaining crêpes.
To finish the crêpes: Spread Nutella inside the crêpe shell, then add in the banana and fold or roll up (save some banana for garnishing the top if desired). Dust with powdered sugar through a fine mesh strainer.
To serve: Serve crêpes with orange juice.