Try adding 1 tsp of vanilla paste to the mixture for extra sweetness.
Heat the waffle maker according to manufacturer's instructions. Whisk the egg whites to stiff peaks. In a separate large bowl, mix together the flour, bicarbonate, caster sugar and a pinch of salt. Make a well in the centre and add the egg yolks and melted butter. Start mixing with a balloon whisk; keep whisking as you slowly add the milk until you get a smooth, thick batter. Carefully fold in the egg whites with a metal spoon.
Heat oven to 180C/160C fan/gas 4. Use a ladle to pour the batter into your waffle maker and cook for 5 mins or according to the manufacturer's instructions. Repeat until all the batter has been used up, placing your finished waffles onto a baking tray as you go.
Once all the waffles are made, transfer to the oven for 5 mins to warm through and crisp. Serve with sweet or savoury toppings.