Scald milk, pour into a large bowl. When lukewarm, add sugar and yeast, mixing well to dissolve yeast. Stir in 1 cup all-purpose flour; cover bowl and set in warm place until "sponge" doubles in volume (about 1-1/2 hrs.).
Combine butter, egg yolks and salt. Stir down the sponge and add butter-egg mixture, along with buckwheat flour and 1 cup all-purpose flour. Beat egg whites until stiff but not dry, then gently fold into batter.
Cook batter, one tablespoonful per blini, on a hot buttered griddle or skillet. Flip blini to brown second side after batter appears set on top. Cover blini with a towel and hold in 200° oven until serving.