Sift the flours in to a large bowl and add the salt and yeast. Heat the crème fraiche and the milk to lukewarm in a saucepan. Whisk in the egg yolks and then add to the flour mixture.
Mix thoroughly and cover with a tea towel. Leave to rise for one hour in a warm place.
Whisk the egg whites in a clean bowl until peaks form, fold into the dough and leave to rise for another hour.
When you are ready to cook the blinis, melt some of the butter in a small hot non-stick frying pan, place a tablepsoon of blini mix in the pan then add a further ½ tablespoon to make the middle thicker. Turn when firm and cook until golden-brown. Cool on rack before serving.