Put the the butter in a microwave safe container and melt it in the microwave for 1 min. Then heat it for an additional 30 seconds so it is hot.
Put the egg yolks, mustard, seasonings and lemon juice in a mixing jug and using a stick blender combine thoroughly.
While still blitzing with the stick blender, add the hot melted butter in a thin stream which should emulsify with the mixture thanks to the lecithin in the yolks – and the mix should thicken.
Once all the butter has been added, remove the stick blender and stand the mixing jug in a pot with a little hot water to keep the jug and it’s contents warm until you use the sauce.