To make the crêpe batter, sift the flour, sugar, and salt into a bowl and set aside; Combine the egg, milk, butter, and vanilla extract in another bowl and stir until smooth; Add this mixture to the dry ingredients and stir until blended into a relatively smooth batter; Let the batter rest for 30 minutes in the refrigerator; (The batter may be prepared to this point and stored in the refrigerator for up to 12 hours).
Heat a crêpe pan or small skillet over medium heat; Brush the pan with melted butter; Pour a scant ¼ cup batter into the crêpe pan, swirling and tilting the pan to coat the bottom; Cook (reducing the heat if necessary) until the first side is set and has a little color, about 2 minutes; Use a thin metal or heatproof rubber spatula to loosen the crêpe, and turn it over; Cook on the other side until set and very lightly colored (the crêpe will not be as dark on the second side as on the first), 1 minute more; Stack the crêpes between layers of parchment or waxed paper as you cook.
To make the cheese filling, purée the cottage cheese and cream cheese in a blender until very smooth; Transfer to a bowl and stir in the sugar, egg, and vanilla extract by hand; Keep refrigerated until you are ready to finish the crêpes.
Combine the blueberries, ¾ cup of the sugar, and one tablespoon of the lemon juice in a saucepan and bring to a simmer over medium heat; Simmer until the sugar has dissolved, about 10 minutes; Taste the mixture and, if necessary, add more sugar; Continue to heat until any additional sugar is dissolved; Strain the coulis through a fine-mesh sieve (Makes 2 Cups).
After straining, place the coulis in a clean saucepan and bring to a simmer; Make a slurry by blending 2 teaspoons cornstarch with 2 teaspoons cold water. Drizzle the slurry into the simmering coulis gradually, adding just enough to lightly thicken the sauce.
Preheat the oven to 400 degrees F; Lightly brush a medium baking dish with melted butter.
Spoon or pipe about 2 tablespoons of the cheese filling onto each crêpe; Fold each crêpe into quarters, or fold the sides in to the center, then roll each crêpe up; Place the crêpes seam side down in the prepared baking dish; Drizzle with melted butter and bake until very hot, 8-10 minutes.
Serve immediately, 2 crêpes per serving, with warm blueberry coulis.