Separate eggs. Mix flour and buckwheat flour in a bowl. Dissolve yeast in a bowl with milk and stir with melted butter and egg yolk. Add yeast mixture and salt to the flour, mix, cover and let rest in a warm place for about 45 minutes.
Then beat egg whites in a bowl until stiff and fold into yeast mixture.
Heat some butter in a pan, add a tablespoon of yeast mixture, flatten slightly and fry on both sides until golden brown. Continue process until all of yeast mixture is completely used. Keep warm.
Spread sour cream on each small pancake, cover with gravlax and top with salmon caviar. Serve immediately.