Heat waffle iron according to manufacturer's instructions.
Preheat oven to 300°F degrees. Place a baking sheet in the oven.
In a medium bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
In a large bowl, whisk together the milk, yogurt, butter, and egg yolks.
Whisk the flour mixture into the wet ingredients until smooth.
In the bowl of a standing mixer fitted with a wire whisk attachment, beat the egg whites until soft peaks form, about 3 minutes. Use a rubber spatula to gently fold the egg whites into the waffle batter until just mixed.
For large Belgian waffles, spread 1 cup of the batter nearly to the edges of the waffle iron. For regular waffles, use ½ cup of batter.
Bake using a medium-high setting for approximately 4 minutes, or until the waffles are deeply golden and crisp.
Serve immediately with maple syrup, or keep warm in the oven until ready to serve.