Serves 4 - 6
4 large free-range egg yolks
250g cold un-salted butter, diced
Put the yolks, butter and two tbsp of water in a heavy based pan and heat very gently, whisking all the time. As the butter melts, the sauce will begin to thicken. Don't be tempted to hurry things along by turning up the heat, the sides of the pan should be cool enough to touch at all times.
Once the butter has melted, turn up the heat to medium-low and whisk vigorously until it thickens: if it begins to steam, take it off the heat, but do not, under any circumstances, cease whisking.
When the sauce is thickened to your taste, stir in one tbsp of lemon juice and season. Taste and adjust if necessary...you may want a little more lemon juice. Serve immediately.
taken from Felicity Cloake: How to make perfect hollandaise sauce.