If the sauce separates or curdles, remove from the heat and immediately whisk in an ice cube. This lowers the temperature and usually saves the sauce. If not, start again with 2fresh yolks, slowly whisking in the curdled sauce until a new emulsion forms.
You should make hollandaise sauce just before serving, but it will keep for about 30 minutes in a bowl set over simmering water if you stir often. Serve with poached eggs, fish and vegetables.
Whisk 3egg yolks with 1teaspooncaster sugar, 1tablespoon water, 1tablespoonwhite wine vinegar and 1tablespoonlemon juice in a heatproof bowl set over a saucepan of simmering water until well mixed and foamy.
Over a very low heat, continue whisking the egg yolk mixture until the whisk leaves a trail on the surface. This should take about 3 minutes. Be careful the bottom of the bowl doesn’t actually touch the water or the yolks may scramble.
Remove from the heat and whisk in 175g (6oz) softened butter, cut into pieces. Add piece by piece at first, then several at a time until the hollandaise sauce is thick but still light enough to pour. Season to taste with salt, pepper and more lemon juice.