For the sponge: Mix the sponge ingredients together in a medium bowl, cover, and set aside to rest for 30 to 60 minutes., For the batter: Whisk together the egg yolks, milk, sugar, and salt. Mix into the sponge and let it rest, covered, for 30 to 45 minutes. While the batter rests, let the egg whites come to room temperature., Just before cooking, whisk the melted butter into the batter. Beat the egg whites until soft peaks form, then fold them into the batter., Preheat the oven to 200°F., Cook the blini on a heated, lightly greased griddle or heavy skillet, using 2 to 3 tablespoons for each blini. Cook as you would pancakes, until the edges look dry and bubbles form that don’t break. Flip and cook the second side until golden brown (about 1 minute). Hold the blini in the oven until ready to serve., Leftover blini can be frozen, layered with wax or parchment paper for up to a month. Reheat in the toaster or in a 300°F oven for 10 minutes.