In a large mixing bowl, whisk together the buckwheat flour, salt and the egg. Gradually add in the 300ml of water (1+1/4 cup), mix to combine.
Cover with the plastic film and refrigerate for at least 2 hours, but preferably overnight.
Just before cooking, add the 50ml of water (1/4 cup) and stir to combine.
Heat a small pan and add the butter to coat. Pour the batter into the pan and spread it evenly. Cook until edges start to turn golden. Turn and cook other side until done. Transfer to the plate.
Enjoy with your favorite fillings!