Warm 1/3 cup of the milk. Dissolve the yeast in the milk. Place in a warm spot without any draught, cover and allow to stand for 10 minutes.
In a bowl, combine the flours and salt and make a well in the centre. Warm the remaining milk and melt the butter. Gradually whisk the egg yolks and yeast into the flour then the milk and butter. Leave the batter covered in a warm place to rest for an hour.
Beat the egg whites until firm and gently fold in the batter. Stand for 10 minutes. Melt a knob of butter in a heavy-based frying pan. When foaming, add a small ladle of batter and cook over a low heat until golden brown. Flip and cook the other side.
Continue until they are all cooked, keeping the blini warm and covered until ready to serve.