Preheat your waffle maker while you prepare the batter.
In a large bowl, mix the flour, baking powder, sugar, and salt.
In another bowl, beat the eggs, then add milk, melted butter, vanilla, and lemon zest if using.
Gradually pour the wet mixture into the dry ingredients, whisking until smooth. Let it rest for 10–15 minutes if you have time (helps with texture).
Lightly grease your waffle iron with melted butter or neutral oil.
Pour in the batter (just enough to cover the bottom plate), close, and cook until golden brown and crisp — usually 3–5 minutes, depending on your machine.