Melt butter and set aside to cool
Whisk egg yolks and vinegar together until thickened and doubled in volume
Place over double boiler with barely simmering water (not boiling, just to the point of some steam escaping), continue to whisk until mixture thickens further and doubles in volume again.
Slowly add melted butter, whisking constantly. If mixture becomes too thick or starts to separate add small amounts of cold water until back to desired consistency
Remove from heat. Add salt, cayenne and lemon juice to taste
Whisk with a small amount of cold water immediately before serving to bring back to desired consistency