Mix buckwheat flour with salt, milk, water, and eggs together. Add melted butter and whisk until you get a smooth batter. Cover with plastic and and let rest for at least 2 hours.
When you are ready to cook the crepes, stir the batter before frying each crepe.
Heat a lightly buttered nonstick skillet over medium high heat.
Add 1/4 cupful batter to skillet and immediately tilt to fully cover the bottom of the pan with a thin layer. Cook for a few seconds until the surface isn't liquid.
Without flipping the crepe, place a slice of ham in the center and sprinkle a little cheese.
A few seconds later, as the cheese is melting and the crepe seems to be ready, carefully lift one side with a spatula and fold it.
Fold the other sides to make a square.
Remove the crepe from the pan and serve quickly.
You can add more grated cheese over the top.