3 egg yolks
1 teaspoon caster sugar
1 tablespoon water
1 tablespoon white wine vinegar
1 tablespoon lemon juice
175 g butter, diced
Whisk the egg yolks, sugar, water, vinegar and lemon juice in a bowl, set over a pan of simmering water, until the mixture is smooth and leaves a trail on the surface.
Remove from the heat, whisk in the diced butter a few pieces at a time.