In a shallow dish combine eggs, yogurt, and salt. Brush butter on one side of all eight pieces of bread. Place one slice of ham, Wisconsin Swiss, and Cheddar cheeses on the buttered side of 4 slices of bread. Top with remaining bread slices, buttered sides facing inward. Melt 2 Tbsp. of butter in a heavy skillet over medium heat. Holding sides of sandwiches, carefully dip in egg mixture and place in heated skillet. Cook until both sides are golden brown and cheese is melted, 3 to 4 min. per side. Add additional butter as necessary. Dust with powdered sugar; serve immediately. Makes 4 servings.