Traditional Method: Whisk 2 egg yolks in a heatproof bowl with 1 dessertspoon of white wine vinegar.
Set the bowl over a saucepan of just simmering water. Make sure that the water does not touch the bowl. Whisk the mixture over the heat for 2-3 minutes until pale and thick.
Melt the butter in a pan and gradually whisk in to the egg mixture a little at a time. If the mixture is too thick add a little lemon juice until you find the right consistency.
Season to taste.
Blender Method: Put the eggs and vinegar into a blender and combine on a medium speed.
Melt the butter then add in a steady stream with the blender on.