Preheat oven to 250°F. Stir together warm water, yeast, and sugar in a bowl and let stand until foamy, about 5 minutes. (If mixture doesn`t foam, discard and start over with new yeast.) Add all-purpose flour, buckwheat flour, and salt, then stir in milk, 3 tablespoons butter, and eggs. Cover bowl with plastic wrap and set in a roasting pan filled with 1 inch of warm water. Let rise in a warm place until dough is increased in volume, has bubbles breaking the surface, and is stringy when scooped, 1 1/2 to 2 hours. Stir batter before using. Heat a 12-inch nonstick skillet over moderately high heat until hot and brush with some of remaining melted butter (if butter browns immediately, lower heat). Working in batches of 4, spoon 1 tablespoon batter into skillet for each blin, then cook, turning over once, until golden on both sides, about 2 minutes. Transfer to an ovenproof platter and keep blini warm in oven. Serve blini topped with sour cream and caviar.