In a large mixing bowl, combine the warm milk, sugar, and active dry yeast, letting it sit for about 5 minutes until the mixture becomes frothy.

Whisk in the buckwheat flour, all-purpose flour, and salt until a smooth, thick batter forms, then cover the bowl with a clean kitchen towel.

Place the bowl in a warm, draft-free area and allow the batter to rise for 45 to 60 minutes, or until it has doubled in size and looks bubbly.

Once risen, whisk the egg yolks and melted butter into the batter until well combined, while in a separate bowl, beat the egg whites until stiff peaks form.

Gently fold the beaten egg whites into the batter using a spatula, being careful not to deflate the air bubbles, which ensures the blini stay light and fluffy.

Heat a non-stick skillet or griddle over medium heat and lightly brush the surface with a small amount of vegetable oil.

Drop tablespoons of batter onto the hot skillet to form small pancakes, cooking for 1 to 2 minutes until bubbles appear on the surface and the edges look set.

Flip the pancakes carefully and cook for another minute on the other side until golden brown, repeating with the remaining batter.

Serve the warm blini immediately, topped with sour cream, smoked salmon, or fresh herbs for a traditional appetizer experience.