Step 1In a large bowl, sift together the buckwheat and bread flours and the salt. Sprinkle with the yeast and set aside.
Step 2In a small saucepan, stir together the milk and creme fraiche. Heat over low heat just until lukewarm, stirring occasionally. Whisk in the egg yolks, then whisk the mixture into the dry ingredients to form a stiff batter.
Step 3Cover the bowl loosely with a damp cloth or plastic wrap and refrigerate overnight. Allow the batter to come to room temperature, still covered, one hour before using.
Step 4Whisk the egg whites to stiff peaks, then gently fold them into the batter with a spatula. Loosely cover the bowl and allow to rise for 1 hour.
Step 5While the batter is rising, combine the yogurt and syrup in a small bowl. Trim and dice the strawberries into one-half-inch pieces and place in a medium bowl. Cover both bowls and refrigerate until needed.
Step 6Heat a heavy cast-iron skillet or griddle over medium heat. Lightly butter the skillet. Scoop the batter into the pan, using a 1-ounce ladle or a generous 2 tablespoons. Spread the batter with the back of a spoon to form pancakes 3 1/2 to 4 inches in diameter. Cook the pancakes 1 1/2 to 2 minutes on each side until golden brown and set. Repeat with all of the batter, buttering the skillet as needed, keeping the cooked pancakes warm.
Step 7Plate three pancakes per serving, topping each with a dollop of sweetened yogurt and strawberries.