Cut the butter into small pats and keep it chilled in the fridge while you prepare other things.
Zest the lemon before you cut and juice. Make sure not to zest the white part of the skin as that part is bitter. Set the zest aside.
Zest the lemon to get the juice and then strain the juice to get rid of any pulps so the curd will be smoother (with an exception of some flecks from the zest, which is supposed to be there).
Put the granulated sugar along with the lemon zest in a food processor or a blender and give it a pulse a few times to make the texture fine. The reason why we do this, so that you get some oil from the lemon zest
Whisk the egg yolks, sugar, lemon juice, and zest in a heavy-bottom saucepan to combine. Put this over medium-low heat, more towards the low and constantly stirring with a spatula until the mixture coats the back of the spatula, about 10 minutes or so. Cook a bit longer if it hasn't coated the back of the spatula.
Remove from the heat and add the butter one pat at a time and keep stirring until the butter melts before adding another one. If you want a REALLY smooth lemon curd, you can strain the mixture to get rid of the flecks from the zest. But I don't mind it. It's your call!
Transfer to a clean jar with a fittin-lid. Cover with a cling wrap, touching the curd directly to prevent skin from forming. Put this in the fridge to let it chill for at least 3-4 hours. Remove the plastic wrap and then secure with the lid. It can be stored in the fridge for about 2 weeks