Combine both flours, the egg and milk into a blender. Using a blender will make sure there are no lumps. The batter should have the viscosity of cream, but not the thickness of waffle/pancake batter.
Any skillet that is 8-10" in width that is seasoned will work. I have used a traditional cast-iron pan, and while it can work, the tall sides makes it difficult for flipping, but it is possible. I then used a traditional crepe pan, and it was far easier to flip with a crepe spatula without fail.
Over medium heat, melt one tablespoon of French salted butter, then pour 1/4 of the batter onto the pan. Using a crepe trowel spreader, spread the batter over the pan to the edges. I should be just enough to cover the pan, but not be thick. If you need to add more batter to fill in gaps, go for it. All will be well.
Cook until the crepe is brown and slightly charred. Then with your spatula - a long semi-narrow one works best. Put the spatula under the middle of the crepe (the crepe will be folding down on both sides, add more butter to the pan to create a nice brown finish on the second side and carefully flip. If the crepe is well-browned on the first side, it will hold together well. There is no need to get fancy and flip carelessly or quickly.
Now that the crepe is cooking the second side, add an egg to the middle of the crêpe, sprinkle a small handful of gruyére and pieces of prosciutto.
Cook the crêpe until the egg is cooked to your liking. If the crêpe is getting too brown and the egg is not cooked yet, fold up the sides to form a square, framing the egg in the middle. Place a lid over the top of the crêpe to more quickly cook the egg.
Place the crêpe onto a dish and serve!