In 1 cup of milk, dissolve the yeast and add 1 teaspoon all-purpose flour. Place the mixture in a warm place until it rises and double in size. Add the rest of the warmed milk. Sift in the remaining all-purpose and the buckwheat flour. Add the sugar, 2 egg whites, and butter. Beat until smooth. Set aside to rise until doubled in size. Knead it lightly, and let it rise again.
Whisk the remaining egg whites until frothy and add to the mixture. Heat a nonstick skillet sprayed with nonstick spray and make 12 pancakes. (These will be thicker than normal pancakes.) Place 2 teaspoons of sour cream on each pancake. Top with caviar.