Lemon Curd

Fill the bottom pot of your double boiler with 1-2 inches
of water. Place on high heat. Once the water begins to boil,
reduce to low heat to keep the water at a simmer.
Place egg yolks, granulated sugar, lemon zest, lemon juice, and
salt into the top pot of your double boiler. Whisk until completely blended, then continue to whisk as the curd cooks.
Constant whisking prevents the egg yolks from curdling.
Whisk and cook until the mixture becomes thick, resembling
the texture of hollandaise sauce, about 10 minutes. If curd
isn’t thickening, turn up the heat and constantly whisk.
Remove pan from heat. Cube the butter, then whisk into the
curd. The butter will melt from the heat of the curd. Pour
curd into a jar or bowl and place a piece of plastic wrap
directly on top so it is touching the top of the curd. (This
prevents a skin from forming on top.) The curd will continue
to thicken as it cools. Once cool, the plastic wrap can be
removed.
Refrigerate the curd for up to about 10 days