In a large mixing bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
In a separate, medium-sized bowl whisk together sourdough discard, milk, egg yolks, and vanilla until well-combined.
Drizzle in melted, cooled butter while whisking (it's OK if the butter forms small clumps).
Pour wet mixture into dry and use a spatula to gently fold together until all ingredients are combined, be careful not to overmix. There will likely be small lumps in the batter.
In a separate, completely clean, dry, bowl, use an electric mixer to beat egg whites into stiff peaks.
Use a spatula to gently fold the egg whites into batter until completely combined.
Allow batter to sit while your waffle iron preheats, about 5 minutes.
Once waffle iron is warmed, pour batter into the center of your iron and cook until golden brown.
Serve warm, topped with butter, syrup, homemade whipped cream, or whatever you desire!
