In a large bowl, dissolve yeast and sugar in water. Stir in milk and let stand 5 minutes.

Sift together flours and salt in a medium bowl. Whisk into yeast liquid. Add buttermilk, oil, and egg yolks, and stir to combine. Cover tightly with plastic wrap and set aside to rise in a warm spot for 1 hour or until mixture is bubbly.

In a small bowl, use a hand mixer to beat egg whites into stiff peaks. Fold into batter, cover, and let rise again for 30 minutes.

Lightly grease a large skillet and set over medium heat. When pan is hot, gently drop tablespoons of batter into pan without crowding. Cook until blini puff and bubble, about 1 minute. Reduce heat to medium-low if skillet starts smoking. Flip blini and cook 1 or 2 additional minutes. Transfer to a baking sheet or plate, cover with a towel, and keep warm in a low oven until ready to serve.

To serve, top each blini with a teaspoon of crème fraîche and a dab of caviar, smoked salmon, or other desired toppings.