Step 11. Begin making the blinis about 1½ to 2 hours before you want to serve. In a large mixing bowl, combine the buckwheat and all-purpose flours along with the salt. Set aside.
Step 22. In a microwaveable mixing bowl, combine the buttermilk and cream, and heat in the microwave until barely warm (about 100 degrees), 60 seconds or so. Using a whisk, beat in the egg yolks, then sprinkle the yeast over the surface of the liquid. Set aside to give the yeast time to rehydrate for a couple of minutes, then whisk it into the liquid and set aside for 10 more minutes.
Step 33. Make a well in the center of the dry ingredients. Pour the liquid into the well and gently whisk until it just comes together into a thick, cohesive batter, being careful not to overmix. Clean the sides of the bowl and cover with plastic wrap. Set aside in a warm place until the surface of the batter bulges and bubbles begin to form and burst, an hour or so (timing will vary depending on the temperature of the room).
Step 44. In a separate bowl, beat the egg whites until they form medium peaks. Gently fold one-third of the egg whites into the batter with a rubber spatula until they are incorporated. Repeat this, folding a third of the egg whites at a time. Once the egg whites are incorporated, set the batter aside to rest for about five minutes before cooking the blinis.
Step 55. For larger blinis, heat a well-seasoned steel or non-stick blini pan (3½-inch diameter) over medium heat. Add a generous spoonful of clarified butter to the pan, then ladle about one-fourth cup (2 ounces) of batter into the pan. Cook the blini until the bottom is golden brown and crisp and the top surface is bubbling. Use a palette knife or thin spatula to turn the blini over. Continue to cook the other side until brown and crisp. Remove the blinis from the pan and repeat the process. Alternatively, you can use a large nonstick pan to cook several small, half-dollar-sized blinis at a time.
Step 66. Though it is best to eat the blinis straight out of the pan, you can place the blinis on a sheet pan in a single layer and keep them warm in the oven. When you are ready to serve, spread a liberal amount of the butter over the surface of the blini and place a generous quenelle or mound of the ice-cold ossetra caviar in the center. Serve immediately with Champagne. Any leftover batter should be made into blinis and then be frozen (they freeze well); otherwise, discard the batter as it will not work well the next day.