Combine egg yolks, sugar, cornstarch, salt, and lemon zest in a medium mixing bowl.
Whisk together well to combine, making sure there are no cornstarch clumps. The egg yolks will also lighten in color slightly.
Add in lemon juice and whisk well.
Transfer the mixture to a pot over medium low heat. Heat the mixture until it thickens, whisking constantly to avoid lumps. It should reach the consistency of a thick custard. Add in the three tablespoons of butter and stir/whisk to combine.
Transfer to a bowl and place plastic wrap so that it touches the surface of the curd, preventing it from forming a skin.
After it has cooled down for a bit at room temperature, put it in the fridge and let it cool completely. Store in the fridge and use within a week!