Heat the milk in a saucepan until lukewarm. Pour on to the yeast and leave for 10 minutes until the mixture is frothy.
Sift the flour and salt into a large bowl. Mix the eggs yolks, cream and yeast mixture into the flour. Set the batter aside for approximately 1 hour, until it has doubled in size.
Whisk the egg whites until they form soft peaks and fold into the batter. Add a little more milk, if necessary, so that the batter is of a thick pouring consistency.
Heat the butter in a frying pan over a medium heat. When it is hot, spoon small amounts of batter into the pan.
Fry the blinis until they are golden brown on both sides and serve at once.