Create a double boiler by bringing 1-2 inches of water to a simmer in a small saucepan. Place a heatproof glass bowl on top and make sure it's not touching the water.
Add the egg yolks, sugar, lemon juice, lemon zest, and salt into the top of your double boiler.
Whisk until completely combined and then continue to whisk as the lemon curd cooks (constant whisking prevents the lemon curd from curdling). Continue to whisk the lemon curd for 10-15 minutes or until it become thick and paler in color (it should be similar to the consistency of hollandaise sauce). If the lemon curd isn’t thickening, turn up the heat and constantly whisk.
Once thickened, remove the glass bowl from the heat. Cut the butter into chunks and whisk into the lemon curd until fully combined and smooth.
If desired, pass through fine mesh strainer and then pour into a glass jar. Let cool to room temperature (about 1 hour) and then cover and store in the fridge for up to 3 weeks.
For thicker lemon curd, replace 2 of the egg yolks with 1 whole egg. This means you will use 4 egg yolks plus 1 whole egg. Keep the rest of the recipe and instructions the same.