Pour warm milk into a large bowl, then sprinkle yeast onto the milk, and allow to soften for 5 minutes. Stir in sugar and buckwheat flour; cover, and let rise for 1 hour.
Stir in the eggs, sour cream, heavy cream, salt, and flour until a batter forms. Cover again, and let rise for 2 hours.
Heat a large skillet over medium heat. Spray with cooking spray, then pour in small amounts of batter to form 1 1/2-inch blinis. Cook until the blinis begin to set and dry, and the bubbles begin to burst, about 2 minutes. Flip over, and continue cooking until browned on the other side, about 1 minute.