Melt 250g butter in a saucepan and skin any white solids from the surface
Put 4 egg yolks, 1 tsp of vinegar and a splash of cold water in a metal or glass bowl which will fit over a small pan. Whisk for a few minutes then put the bowl over a pan of barely simmering water and whisk continuously until pale and thick (3-5 minutes).
Remove from heat and slowly whisk in the melted butter, bit by bit until it is all incorporated and you have a creamy hollandaise. If it gets too thick add a splash of water. Season with a squeeze of lemon and a little cayenne pepper. Keep warm until needed.