In a steel bowl, whisk together all the ingredients except the butter.
Bring a pot of water to a slow simmer and put the steel pbowl on it, whisking all the way. YOU DONT WANT IT TO CURDLE. Of course, you need to make sure the size of your pot and the steel bowl is such that it fits right inches.
Once the mixture thickens, stirring pretty constantly, to coat the back of a spoon, take it off the pot. Add in the slab of butter and whisk well so that the butter melts into the mixture. Let cool and transfer to a clean jar. Put cling film on it before closing the lid and refrigerate overnight. It keeps in the refrigerator for a week.