Sprinkle yeast over hot water. Stir and allow the mixture to cure for 5 minutes until yeast is bubbly. Blend yogurt with sugar, salt and oil. Stir into the yeast mixture. Add buckwheat and beat until batter is smooth.
Cover the bowl and allow to stand in a warm, draft-free place for 20 minutes. Meanwhile, whip egg whites until soft peaks form. Fold the last cup of warm water into the batter, fold in egg whites and let stand for 10 minutes.
Preheat the oven to 250 degrees (for warming cooked Blini). Spray a Blini pan or a 6-inch crepe pan with non-stick cooking spray. Heat the pan over medium heat. Place a scant ¼ cup batter in the pan and cook until top is firm and bubbly. Turn Blini and cook until crisp and brown, about 5 minutes. Warm in the oven until ready to serve.
You may also store Blini, covered in the refrigerator for up to 3 weeks or wrap and freeze them for up to 3 months. To reheat, microwave each Blini for 20 seconds (high), or heat in a 350 degrees oven for 5 minutes.