Start with a double boiler and start with first 3 ingredients.
Continuously stirring until it reaches 141 degrees on the candy thermometer.
Carefully Pour mixer into a electric stand mixer and using a whisk, mix until medium peaks appear.
Once it has cooled and is a medium to stiff meringue stop the mixer.
Change to the paddle attachment and start mixing on low while adding the butter in small pieces.
Then start mixing on medium speed and add vanilla.
If will start to look runny and after a few minutes it will look like curdled milk and then in will start to thicken. This takes about 10 minutes in the mixer. Once its done it should hold its shape on a spoon.
If at any point you need to thicken it up you can always put the bowl into the refrigerator for 5 - 10 minutes. It will thicken if you keep beating.