Bring a pot of water to a boil and then turn it down to a simmer.
Grab a glass bowl and put it over the pot of water (so it snugly fits).
Add all the ingredients (except for the butter) and whisk for five minutes.
Next take it off the heat for 30 seconds and whisk, then put it back on the heat and whisk it for an additional 30 seconds. You will repeat that for 5-6 minutes, or until the sauce thickens up.