Fill ⅓ of the bottom pot of your double boiler and place on high heat. When your water begins to boil, reduce heat to medium-low to keep the water at a simmer as you are cooking your lemon curd.

Put lemon juice, lemon zest, sugar, and salt into the top pot of your double boiler. Whisk until the ingredients are well combined and continue to whisk well throughout the cooking process. Constant whisking prevents the egg yolks from overcooking and becoming scrambled eggs.

Whisk until the mixture becomes thick, and coats the back of a spoon with a thick layer, about 12-15 minutes. If your curd isn't thickening fast enough, slightly turn up the heat while whisking constantly. The whisking is the key to smooth, non-scrambled, lemon curd.

Remove pan from heat and stir in butter, a cube or two at a time, letting butter melt before adding another cube.

Pour curd through a mesh sieve to ensure a smooth curd into a bowl, then place a piece of plastic wrap over, touching the curd so it doesn't form a skin. It will continue to thicken as it cools down.

Store in the fridge up to 10 days or freeze for up to 6 months and thaw in fridge to enjoy.