Start by preheating a waffle iron. You may also decide to preheat the oven to 200℉ to keep the waffles warm if they won't be served right away.
In a large bowl, combine the flour, sugar, baking powder, baking soda, and salt.
In a separate bowl or large measuring cup, combine the buttermilk and egg whites and mix until incorporated.
Melt and let cool unsalted butter, then beat in the egg yolks.
Next, combine all the wet ingredients: sourdough discard, the buttermilk and egg white mixture, and the butter egg yolk mixture.
Add the wet mixture to the dry ingredients and stir until combined; do not overmix. Let the batter sit, covered for 10 minutes.
Brush the heated waffle iron with oil. Add the batter to the center of the waffle iron and spread the batter it so it's somewhat even. One-quarter cup of batter for each 4-inch square on the waffle maker is a good place to start when trying to figure out how much batter to use.
Cook the waffles until they are a mild to medium brown on the outside and cooked through on the inside. Cook according to the manufacturer’s instructions, typically around 4-6 minutes, but cook times and batter amounts will vary depending on the size and type of waffle iron. *The batter will usually create a steady stream of steam that can be seen escaping from the sides of the waffle maker. When the waffles are done, that steam will have slowed down quite a bit. That can be a good indicator that waffles are done and ready to be taken out. Waiting until there is no steam usually means they are overly done and can end up being dry or burnt.
To keep the waffles warm while others are cooking, heat an oven to 200 degrees and transfer the waffles to a baking sheet, uncovered to keep them warm and crisp until they are ready to serve.