1.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.
2.Place egg yolks, sugar, lemon zest, lemon juice, and salt into the top pot of your double boiler. Using a silicone whisk, 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.
3.Remove pan from heat. Cut the butter into 6 separate pieces, then whisk into the curd. The butter will melt from the heat of the curd. Pour curd into a jar 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.
4.Refrigerate the curd for up to about 10 days.