Combine yeast, sugar, and warm water in a 1-cup glass measuring cup; let stand 5 minutes.
Combine flours and salt in a large bowl; make a well in center.
Beat egg yolks lightly in a small bowl; add yeast mixture, milk, and butter. Add to dry ingredients, stirring with a whisk until mixture is smooth. Cover with a tea towel. Let mixture stand at room temperature 1 1/2 to 2 hours, or until batter is light, bubbly, and doubled in bulk.
Stir mixture to eliminate air bubbles.
Beat egg whites at high speed with an electric mixer until stiff peaks form; fold into batter.
Drop blini batter by tablespoonfuls onto a hot, lightly greased griddle.
Cook until tops are covered with bubbles and edges look cooked; turn and cook until lightly browned. Keep warm. Serve with sour cream and your selection of caviar.