Prepare a custard base by combining all ingredients except the vanilla in the top of a double boiler. Gradually heat mixture to 74º–82º C. The water in the bottom of the double boiler should be simmering and your stove should be on a low setting. Hold at this temperature for 10–15 minutes. The mixture should smoothly coat the back of a spoon.
Quickly cool the custard in an ice bath or in a sink of cold water. Add the vanilla extract.
Cool the mixture in a refrigerator for several hours or overnight.
Fresh mozzarella is best enjoyed, well, fresh, but can be wrapped tightly in cling film and stored up to 1 week in the refrigerator or frozen up to 3 months.