Melt the butter in a saucepan.
Heat 1.5 - 2" water to a simmer in another pot. The bowl in the next step needs to fit snugly on top, and not touching the water.
Whisk egg yolks in a small bowl until frothy.
Whisk in everything other than the butter.
Place the bowl on top of the simmering water, and keep whisking until the mixture thickens a little and turns a pale yellow.
Remove the bowl from the heat, and slowly whisk in the melted butter until the sauce is smooth.
If you see any lumps at all, pour through a strainer.
Serve immediately.