Fill a saucepan with some water and place a large bowl on top of it, making sure it’s not touching the water. Bring the water to a low simmer.

Add the lemon juice, zest, egg yolks and whole eggs, and sugar to the bowl. Whisk to combine, then continue to stir constantly as the curd thickens. Take your time - if you cook the curd too quickly, you could end up with scrambled eggs and nobody wants that. Keep stirring gently until the mixture reaches 165°F and is thick enough to coat the back of a spoon. This process will take 10-20 minutes.

Remove the bowl from the heat and fold in the butter. Keep mixing until the butter is fully melted.

If desired, run the hot curd through a fine-mesh strainer to remove any lumps.

Refrigerate for 1 hour. The lemon curd will continue to thicken as it cools.