Melt the butter in a pan or microwave and set aside until needed.

Put 2 inches of water in a saucepan and heat very gently so it is barely simmering.

Separate the egg whites from the egg yolks.

In a heatproof bowl, add the white wine vinegar to the egg yolks and whisk until light and frothy.

Place the bowl on top of the saucepan, make sure that the base of the bowl doesn't touch the water.

Whisk the mixture constantly as it is sitting over the simmering water until it thickens slightly. (Don't overheat or you'll end up with scrambled eggs!)

Take the mixture off the heat completely and start to whisk in the melted butter a little at a time.

Add the lemon juice bit by bit, tasting as you go, until you get the flavour you like.

Season with salt and black pepper.