Put the egg yolks and a little water into a small pyrex bowl and rest it over a pan of barely simmering water, bain marie style.
Whisk constantly with a balloon whisk until light and frothy, then remove from the heat, still whisking. Start to trickle the butter in (see recipe), whisking all the time until you have a thick, smooth consistency.
Pour in a splash of the reduction - about 1 tsp - to keep it loose. If the sauce is too thick, thin down with a little hot water. Season to taste with salt and pepper.