Separate the egg yolks and whites. Whisk the yolks until creamy, then add yogurt and melted butter until smooth.

Sift together flour, baking powder, and salt in another bowl. Fold this mixture into the wet ingredients gently.

Fold in cooked spinach and crumbled feta cheese until evenly distributed.

Beat the reserved egg whites until soft peaks form and gently fold them into the batter.

Preheat the waffle iron and lightly grease it. Pour batter into the iron, filling about two-thirds full and cook for 5-7 minutes.

Carefully remove the waffles and serve warm with toppings like salsa or sautéed mushrooms.