Preheat oven to 300°. Heat waffle iron until very hot. Whisk flour, sugar, baking powder, baking soda, and kosher salt in a large bowl. Using an electric mixer, beat egg whites in a medium bowl until medium-soft peaks form. Whisk egg yolks, melted butter, buttermilk, and soda water in a medium bowl; gradually whisk into dry ingredients. Fold in egg whites.
Coat waffle iron with non-stick vegetable oil spray. Pour batter onto iron, spreading it into corners (amount of batter needed will vary according to machine). Scatter 1 rounded Tbsp. ham and 1 Tbsp. white cheddar over each waffle. Cook until golden brown and cooked through. Transfer to a baking sheet; keep waffles warm in oven between batches. Serve with butter and warm maple syrup.
Courtesy of Bon Appetite magazine ... this just looked like YUM! to me! Of course, the crispy little cheese bits... and all that butter... it has to be good! savory and sweet.