Add milk, flour, sugar, salt, butter, vanilla, and eggs to a high-powered blender. Blend on low speed until well combined, about 10–15 seconds.
Scrape sides and bottom of the blender with a spatula and blend a few seconds longer. Cover and refrigerate for at least 1 hour (or overnight if desired).
Heat an 8-inch, nonstick pan over medium to medium-low heat. Set out a large tray. Cut small squares of parchment or wax paper to separate crepes. Remove the batter from the fridge and give it a quick stir.
Lightly butter the pan with melted butter and a pastry brush or paper towel dipped in the butter. Barely lift the pan off the heat and pour 1/4 cup batter into the pan. Immediately, and working quickly, tilt and swirl the crepe pan in a circular motion to evenly coat the bottom of the pan with batter. Return pan to the heat.
Cook until the crepe is golden brown on bottom, 1–2 minutes, use an offset spatula to lift an edge of the crepe. Gently grab that edge and use your fingers and the spatula to flip the crepe to the other side. Cook until golden brown spots appear on the bottom, another 1–2 minutes. Remove to the prepared tray. Place a piece of parchment or wax paper on top of the crepe.
Repeat until all the batter has been used. If crepes are cooking too quickly, reduce heat as needed. Increase the heat if they are taking longer than 1–2 minutes per side to cook.
Custard
In a small pot, combine the sugar, cornstarch, and salt. Heat on medium-high heat. Gradually add in the milk while whisking constantly. Cook and stir until thickened and slightly bubbly, about 3–5 minutes.
Reduce heat to low. Cook and stir for 2 minutes. Remove from the heat and let cool for 1 minute.
Meanwhile, place 3 room-temperature egg yolks in a small heat-safe bowl. Stir a small amount of hot milk filling into egg yolks while whisking constantly. Pour the egg yolk mixture into the pan, stirring constantly. Return the pan to medium heat and bring to a gentle boil; cook and stir for 2 minutes. Remove from heat and stir in vanilla. Cover and chill for 1 hour.
Assembly
Place 1 crepe on a plate. Spoon the custard on the bottom half of the crepe and top with thinly sliced bananas and a sprinkle of chopped walnuts. Fold the crepe in half and then in half again to form a triangle.
Top with desired toppings—whipped cream, maple syrup, heart-shaped banana slices, or finely chopped walnuts (see note 2). Enjoy immediately.
Notes
Note 1: For best results, do not use lower fat milk.Note 2: Instead of maple syrup you can top these crepes with this "caramel" syrup.Nutrition Note: The nutritional information includes the custard, but not the optional whipped cream and syrup.Storage: You can stack the cooked (completely cooled) crepes with parchment paper, place them in a large resealable plastic bag, and refrigerate them for 3–4 days.