Preheat your Belgian waffle iron on the hob over a medium heat; if using an electric waffle maker, heat according to the manufacturer’s instructions. You can make these waffles with a standard waffle iron, but the mixture will make more (thinner) waffles.
Sift the flour, baking powder, caster sugar and a pinch of salt into a large mixing bowl and make a well in the middle.
In a jug, whisk together the milk, egg yolks, melted butter and vanilla until smooth. Pour the milk mixture into the dry ingredients and whisk until smooth and thoroughly combined.
In a clean bowl, whisk the egg whites with a pinch of salt until they hold firm, but not stiff, peaks. Using a large spoon, fold the egg whites into the batter mixture until just combined but do not over-mix.
Spoon the batter into the waffle iron – how much you need will depend on the size of your iron or machine – and cook on the hob for 2–3 minutes on each side until golden and well risen, or according to the manufacturer’s instructions. Keep warm while you cook the remaining waffles.