How to Make It
In a bowl, whisk together the flours, sugar, soda, and 1/4 teaspoon salt. Whisk in the milk and the egg yolk. In a separate bowl, whisk the egg white until it holdS soft peaks. Gently fold the egg white into the batter until combined. Fold the 2 tablespoons melted butter into the batter.
In a large heavy cast iron or non-stick skillet, heat about 1/2 tablespoon of butter over medium heat until hot. Working in batches, spoon level 1 tablespoons of the batter (about 6 at a time) into the skillet and cook until bubbly and dry on top, about 1 minute. Flip the blini and cook until golden on the bottoms, about 45 seconds more. Transfer the blini to a serving platter and repeat with the remaining batter.
Serve the blini with sour cream, dill, and caviar, if using.