Prepare your station. On the stove, have a pot of water simmering. Nearby, have an empty clean bowl set up with a fine-mesh sieve over the top; set aside.
In a different medium heatproof bowl, whisk together the caster sugar and egg yolks - being sure not to leave them sitting together for too long before beating. When thoroughly combined, whisk in the lemon juice + zest and set over the prepared pot, ensuring the bottom of the bowl doesn't touch the water. Whisking occasionally, leave the curd to cook until the temperature registers 82°C on a digital thermometer, or it is thick enough to coat the back of a spoon, about 15-20 minutes.
Strain the curd into the sieve set over the clean bowl.
Begin to add the butter into the curd, a few cubes at a time. Use a rubber spatula to stir everything together - the heat from the curd should easily incorporate the room temperature butter.
Sieve the curd again, this time into a storage container. Press cling film directly onto the surface of the curd to prohibit a skin forming. Refrigerate until set before using, about 3-4 hours.