To prepare the Belgian Waffle batter
Mix the flour, sugar, oil and baking powder in a bowl. Add the 3 egg yolks and mix. Gradually incorporate the milk to avoid creating lumps.
Beat the egg whites into stiff peaks and blend them into the batter.
Let the batter rest for about 1 hour before cooking.
Cooking of the Brussels waffles
Preheat the waffle iron to 200 °C for 15 minutes.
Before use, give the batter another gentle mix to make it uniform.
Brush the irons with a light coat of oil. Use a ladle or jug to fill the irons on one side until the tops of the grid outlines are covered. Close the waffle irons and lock into position using the handles. Turn the irons over immediately (for 180° waffle irons and swivel waffle irons).
Cook in this position for 3 to 4 minutes.
Open the waffle iron and remove the waffles using a waffle pick. It is best to place the waffles on a rack to prevent them from going soft.