Place a pot with water over medium heat. Once it starts to boil reduce the heat until you see only a slight simmer and place a bowl on top. Make sure that the base of the bowl isn't touching the water in the pot.
Add the lemon juice, zest, eggs and sugar to the bowl. Whisk everything together. Mix every couple of minutes with a spatula until the curd cooks. It's ready when you can see it's more gelatinous. Also, if you draw a shape with the mix on its surface it will stay there. This will take 15-25min. You want to avoid overcooking the curd or it could curdle.
Remove from the heat and transfer to another bowl through a sieve or colander. The colander will catch large pieces of zest and also any egg that cooked funny. Changing it to another bowl will also stop the cooking process. Add the butter and mix until fully melted and incorporated.
Let cool completely to room temperature and then refrigerate overnight before using.