½cupbutter1 stick, cut into small pieces and kept cold
Instructions
Add enough water to a medium saucepan to make the water 1-inch high in the pan. Place over medium-high heat and bring the water to a simmer.
In a glass or metal bowl, whisk together the egg yolks and sugar until combined and smooth.
Measure 1/3 cup of lemon juice. If you don't have enough lemon juice to make a full 1/3 cup, add water until the measurement is reached. Add the lemon juice and lemon zest to the egg yolk mixture. Whisk until smooth.
Place the bowl over the steaming water (creating a double boiler). Cook the lemon mixture for 8 to 10 minutes, or until the sugar has dissolved, the mixture is quite warm, and the liquid coats the back of a spoon.
Remove the lemon mixture from the heat. Add 1-2 pieces of cold butter to the lemon mixture and whisk until almost completely melted. Add a 1-2 more piece of butter and whisk to melt. Continue until all the butter has been added and is melted. (DO NOT add all the butter at once. You want to emulsify the fats into the lemon mixture.) You will feel the lemon curd thicken as you add the butter.
Transfer the lemon curd to a clean bowl and cover with plastic wrap - the plastic wrap should be touching the top of the lemon curd to prevent a skin from forming.
Chill in the refrigerator for 1-2 hours until the curd is cooled and thickens more. Transfer to an airtight container.
Notes
This lemon curd will keep for up to 2 weeks in the fridge.