Combine the milk, butter, eggs and salt in a blender. Pulse a few times to combine.
Add the buckwheat flour and pulse again just until blended and then thin with water to the right consistency.
Pre-heat a 10-inch skillet or crêpe pan over medium heat.
Brush the pan with a little melted butter and then pour in enough batter to just cover the bottom of the skillet. If you have any excess batter in the pan, pour it back out into the batter bowl. (Measuring with a ¼ cup measure is a great way to be consistent.)
Reduce the heat to medium and cook the crêpe for 1 minute on one side. Loosen the edge of the crêpe with a silicone spatula and then grab the edge with your fingers, lift it out of the pan and flip it over. Cook for another minute on the second side and then turn it out of the skillet onto a plate.
Repeat for as many crêpes as you need, or until you run out of batter, monitoring the temperature under the pan to keep the crêpes from burning or not cooking quickly enough.
You can make these crêpes dairy-free if you eliminate the butter (or use ghee) and replace the milk with water. The milk does add flavor to the crêpes, but since you are probably filling the crêpe with a flavorful savory filling, you will be fine. I've had it both ways and never complained!
In addition, you can use some hard apple cider or sparkling water instead of the ¼ cup of water at the end of making the batter. The apple cider (which is the classic pairing with Galettes Bretonne) definitely adds a sparkly note of flavor to the crêpe.