Heat up about 2 inches of water in the bottom of a double boiler.
Add the egg yolks and sugar to the top pan of the double boiler and whisk well to combine. Whisk in the lemon juice and place it on top of the steam, making sure the bottom doesn't touch the simmering water below.
Continue to gently whisk for a few minutes making sure the curd doesn’t stick to the bottom. About 10 minutes, until it thickens up enough to coat the back of a spoon.
Remove pan from the steam and whisk in the cold butter until melted. Add the lemon zest and stir to combine.
Transfer to a bowl and cover with plastic wrap, making sure it touches the surface. This will prevent the curd from forming a crust. Refrigerate until ready to use.
Use as a filling this Italian Lemon Olive Oil Cake ,or serve chilled in dessert bowls with blackberries and a dusting of powdered sugar.
Notes
FAQ: Can You Freeze Fresh lemon Curd ?A: Yes! Follow the above recipe and place the curd in a freezer container. Once set simply freeze for up to 1 year. To defrost transfer the container to the refrigerator a day before you want to use it.