Pour the milk into a bowl or glass measuring cup. Stir in the lemon juice. Let stand for 5-10 minutes at room temperature to make sour milk.
Beat the egg whites in a medium mixing bowl with a mixer on high speed until stiff peaks form. Do not overbeat. Set aside.
In a large mixing bowl, whisk together the flour, baking powder, salt, sour milk, egg yolks, melted butter, brown sugar, and vanilla until well combined.
Use a rubber spatula to gently fold in beaten egg whites.
Preheat a waffle iron and spray the plates with non-stick cooking spray. Spoon about 3/4 cup of batter onto the waffle iron, spreading it out towards the edges.
Close the lid and cook until golden brown. Remove from the iron and butter immediately. Repeat until all of the batter has been. Serve warm with maple syrup.