In a medium bowl, whisk together the flour, sugar, baking powder and salt. In another bowl, whisk together the milk, egg yolks and melted butter. Pour the wet mixture into the dry and stir until just combined.
Preheat a griddle or large nonstick skillet over medium heat. Meanwhile, beat the egg whites to soft peaks. Gently fold the whites into the batter.
Grease the griddle and ladle the batter in 1-tbsp. increments to make blinis. Let cool on a wire rack. Top each blini with sour cream, 1 slice smoked salmon, 1 slice avocado and 1 sprig dill.
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