Blend: Place the flour, sugar, salt, milk, mashed banana, eggs, melted butter, and vanilla in your blender. Process until completely smooth, about 20-30 seconds. If possible, let the batter rest for at least 30 minutes in the fridge. Overnight is even better — the batter will be perfectly hydrated and any bubbles will settle.
Heat: Place your 7- or 8-inch nonstick skillet over medium heat. Let it get hot — about a minute. Coat the pan with a thin layer of butter. If the butter immediately sizzles and foams, you're at the right temperature.
Swirl: Pour about ¼ cup of batter into the center of the pan. Immediately lift the pan off the heat and tilt and swirl it until the batter evenly coats the bottom. If you're using a 7-inch pan, use slightly less than ¼ cup — you want them thin.
Cook: Let it cook for about 1 to 1 ½ minutes. When the bottom is golden brown and the edges start pulling away from the pan and look lacy and dry, it's ready. Use your fingers or a spatula to gently pull the edges up.
Flip: Flip the crepe confidently. Cook for another minute on the second side. Transfer it to a plate. You can stack them right on top of each other — they won't stick.
Repeat: Continue with the remaining batter. Add a tiny pat of butter to the pan between each crepe to keep them from sticking and to maintain that golden color.
Fill: Take a cooled crepe and spread a generous layer of Nutella over one half. Top with a few slices of fresh banana. Fold the crepe over to enclose the filling, then fold it in half again into a quarter-circle.
Serve: Dust generously with confectioners' sugar and add a few extra banana slices on top right before serving.