Side

Whisk buckwheat, all-purpose flour, salt, and yeast in a large bowl
Make a well, add warm milk, and whisk until smooth
Cover and let the batter rise until doubled, about 1 hour
Stir melted butter and egg yolk into the batter
Whisk the egg white until stiff in a separate bowl
Fold the whipped egg white into the batter
Cover and let the batter stand 20 minutes
Heat butter in a nonstick skillet over medium heat
Drop quarter-sized dollops of batter into the pan without overcrowding and cook until bubbles form, about 1 minute
Flip and cook about 30 seconds more
Transfer blini to a plate and cover to keep warm
Add more butter to the pan and repeat frying with the remaining batter