Whisk sugar, cornstarch, salt, and the zest of two lemons together.
To the sugar mixture, add the whole eggs and egg yolks and whisk until smooth.
Zest the last two lemons and set the zest aside. Stir in all of the lemon juice into the sugar/egg mixture.
Cook gently over medium-low heat, stirring constantly until it thickens and reaches 180F.
Remove from heat, whisk in cold butter gradually into the warm curd, and whisk gently until glossy. Add in the remaining zest we set aside.
Strain if desired and chill with plastic wrap touching the surface. It will thicken as it chills in the fridge. Use within the first few days.