Fill the bottom pot of your double boiler with 1–2 inches of water. (Or use the DIY double boiler method listed in the notes.) 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. 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 you’d like to be precise and use a candy or instant-read thermometer, the temperature will rise to about 170°F (77°C). If curd isn’t thickening, turn up the heat and constantly whisk.

Remove pan from heat. Whisk the sliced butter into the curd. The butter will melt from the heat of the curd as you whisk. 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.