Combine the lemon juice with the sugar in the small pot then bring to a boil. Remove from heat and temper eggs by slowly pouring 3/4 of the boiling liquid into the yolks while constantly whisking.
Return all the liquid back to the pot and whisk constantly over low heat until thickened. Remove from heat and mix in cubed butter until fully incorporated.
Strain curd into an airtight container then press a layer of plastic wrap to the top (to prevent a skin from forming). Cover with lid and refrigerate for up to a week.