Mix buckwheat flour with salt, milk, water, and eggs together. Add melted butter and whisk until you get a smooth batter. 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.
Without flipping the crepe, place a slice of smoked salmon in the center, add 1 Tsp of sour cream, and sprinkle with dill.
A few seconds later, as the crepe seems to be ready, carefully lift one side with a spatula and fold it to make a triangle, or a square.
Transfer the crepe to the plate and serve right away.