In a large bowl, whisk together 1/2 cup buckwheat flour, 2/3 cup all-purpose flour, 1/2 teaspoon salt, and 1 teaspoon yeast. Make a well in the center and pour in 1 cup warm milk, whisking until the batter is smooth. Cover the bowl and let the batter rise until doubled, about 1 hour. Enrich and Rest the Batter Stir 2 tablespoons melted butter and 1 egg yolk into the batter. In a separate bowl, whisk 1 egg white until stiff, but not dry. Fold the whisked egg white into the batter. Cover the bowl and let the batter stand 20 minutes. Pan-Fry the Blini Heat butter in a large nonstick skillet over medium heat. Drop quarter-sized dollops of batter into the pan, being careful not to overcrowd the pan. Cook for about 1 minute or until bubbles form. Turn and cook for about 30 additional seconds. Remove the finished blini onto a plate and cover them with a clean kitchen towel to keep warm. Add more butter to the pan and repeat the frying process with the remaining batter.