Fill a saucepan with about an inch of water. Set a stainless steel or glass bowl over saucepan that fits nicely without touching water. Bring water to a simmer.
Add cream, egg yolks, lemon juice, mustard, and cayenne pepper to bowl over saucepan whisk well. Cook mixture, whisking constantly, until just thick enough to coat the back of a metal or wooden spoon and temperature reaches 160 degrees.
Reduce heat to very low, then very slowly drizzle in warm melted butter while whisking constantly. Remove from heat, season with salt and pepper to taste. Transfer to dish, cover to keep warm.
