2 egg yolks
110g butter, cubed
1 tbsp cold water
Approx 1 tsp lemon juice (to taste)
Pinch of salt
Put the egg yolks and water in a glass heatproof bowl (you can use a metal bowl at a pinch) over a bain-marie of simmering water.
Whisk them together until slightly frothy. Start by adding 1 piece of butter at a time, when one melts add another, whisking all the time.
Be careful that the eggs don't get too hot, or they'll curdle. You can always dip the bottom of the bowl in cold water if you think the sauce is beginning to overheat.
You can then add a few pieces of butter at a time whilst whisking.
Once all the butter is in the sauce, season with salt and lemon juice.
Remove from the heat and keep warm (hollandaise can't be reheated). You can use a thermos flask or a keep the sauce in a jug sitting in a pan of hot water until needed.