Melt over low heat:
Keep warm. Place in a large stainless steel bowl or the top of a double boiler:
Off the heat, beat the yolks with a whisk until light and frothy. Place the bowl or the top of the double boiler over barely simmering water and continue to whisk until the eggs are thickened, 3 to 5 minutes, being careful not to let the eggs get too hot. If needed, remove the bowl or pan and whisk to cool the mixture slightly. Whisking constantly, very slowly add the butter, leaving the white milk solids behind. Whisk in:
If the sauce is too thick, whisk in a few drops of warm water. Serve immediately or cover and keep warm for up to 30 minutes by placing the bowl or pan in warm (not hot) water.