Beat egg yolks, add yogurt, salt and cornmeal.
Beat egg whites stiff and fold them into cornmeal mixture.
Heat 10-inch cast-iron skillet medium-hot, melt butter in it and pour in cornbread batter.
Cover skillet and keep over medium-hot fire or burner for 10 minutes.
Carefully loosen cornbread around the outside, cut into wedges and turn each wedge over.
Cover skillet and let cook another 2 minutes or so, until top of cornbread has browned.
Remove from skillet and serve hot.