Cream together the yeast, sugar and lukewarm milk in a jug. Leave to become frothy.
Place the plain flour in a bowl, make a well in the centre and beat in the yeast mixture. Leave in a warm place to expand, about 30-60 minutes.
Mix in the buckwheat flour and egg yolks and leave to prove for another 30 minutes.
Whisk the egg whites with a pinch of salt until stiff, then fold into the batter.
Heat a little butter in a frying pan. Drop 15ml spoonfuls of batter into the pan, spacing apart, and cook for 2-3 minutes until the bubbles start to appear at the top, and the underside is golden. Turn over and cook on the other side. Place on a rack.