Heat the oven to 110C/fan 90C/gas ¼. To make the hollandaise, whisk the yolks, 1 tbsp vinegar and a pinch of salt in a heatproof bowl. Set over a small pan of gently simmering water (make sure the bowl isn’t touching the water). Whisk for about 3-5 minutes over the heat until pale and thick. Add cubes of butter, 2 at a time, to the bowl, continually whisking, and wait for this to be incorporated until you add more. Keep adding the butter, and whisking until the sauce thickens, and turns into a thick hollandaise. Add the lemon juice, and keep warm until needed, giving it a whisk every so often so it doesn’t form a skin.