Melt the 6 Tbsp. of butter and set aside.
In a large bowl, whisk together flour, sugar, baking powder, baking soda and salt.
In a separate bowl, whisk together wet ingredients: yogurt, milk, melted butter and eggs. Fold wet ingredients into dry ingredients.
Preheat waffle iron and using a pastry brush or paper towel lightly coat top and bottom of waffle iron. Using about 1/2 cup batter per waffle, cook the waffles until golden and crisp.
Serve immediately or keep warm in a 200 degree oven until ready to serve.