In a medium saucepan, whisk together sugar, lemon juice, eggs, egg yolks, and lemon zest.
Heat over medium low, whisking at all times.
After cooking and whisking for 5-8 minutes, the mixture should begin to thicken. If it does not thicken, turn the heat up just a bit, but do not turn heat above medium.
Once the mixture thickens, remove it from the heat and whisk in the pieces of butter until melted and combined.
If you would like extra smooth lemon curd, you can pass the curd through a fine-mesh sieve before transferring it to containers. Transfer the hot lemon curd to clean canning jars or a heat-proof bowl. Press plastic wrap onto the surface of the curd to prevent it from forming a skin while cooling.
Cool down the curd to room temperature and then chill for at least one hour before using. Remove the plastic wrap after the curd has cooled.
Notes
Regular lemons or Meyer lemons can be used in this recipe.
This lemon curd freezes well. You can easily make double and store any extra in the freezer for later use.
Don't use bottled lemon juice. Freshly squeezed juice from a lemon is always best.