Preheat a waffle iron. Into a large bowl, sift together buckwheat and all-purpose flours, sugar, baking powder, salt, and cinnamon.
In a medium bowl, whisk together egg yolks, sour cream, milk, and butter. Pour the egg mixture into the dry ingredients, and stir briefly until just combined.
In a medium bowl, beat egg whites with a hand-mixer until stiff but not dry. Fold into the batter.
Ladle about 1/3 cup batter onto each section of the waffle grid; spread the batter almost to the edges. Close the lid, and bake about 5 minutes, until no steam emerges from the waffle iron.
Transfer the cooked waffles to a baking sheet; place in an oven set to low heat, about 200 degrees, while using the remaining batter. If desired, serve with blueberries and a dollop of creme fraiche.