In the bottom of a double boiler (see notes if you don't have a double boiler) bring water to a boil.
When water is boiling, add egg yolks, sugar, lemon juice, and lemon zest to the top portion of the double boiler and whisk until smooth. (We recommend using a glass bowl on top and a silicone whisk).
Continue whisking gently for 10 to 20 minutes, until the mixture has reduced by about half and begun to thicken. (It will be done when it will coat the back of a spoon without dripping off).
Remove from heat, and gently stir in the softened butter. Pour the mixture through a fine mesh sieve and dispose of remnants.
Transfer curd to an air tight container and chill for at least 3 hours.
Notes
RECIPE MAKES ABOUT 1 CUPCalories are automatically generated. For best results calculate based on your exact ingredients.How to make lemon curd without a double boiler
You can use a saucepan if you have no access to a double boiler, but we strongly prefer the double boiler method as you're much less likely to scorch the mixture.
To create a makeshift double boiler, just place a heat proof glass bowl over a pot of boiling water (making sure the bottom of the bowl is not touching the water).
We do not recommend using a metal bowl or metal double boiler.