In a large bowl mix the yeast and warm water and allow to sit for 5 minutes. Whisk in the whole milk and melted butter. Add the all-purpose flour, buckwheat flour, and salt, and mix well. Cover and let the mixture to rise in a warm place until doubled in size, about 2 hours.
When the dough has risen and doubled in size add the buttermilk and fold in the whipped egg whites until well mixed.
Heat a large skillet over medium heat and coat with the cooking spray before beginning to make the pancakes. Add tablespoons of batter to the skillet, forming 1-inch diameter pancakes (or larger, if you like) and cook until golden brown on 1 side then flip them over and do the same on the other side. Remove from pan allow to cool on a platter. Repeat with remaining batter. Top with creme fraiche and caviar and serve.