Add the sugar, egg yolks, lemon zest, and lemon juice to a heat-safe bowl or double boiler. Whisk to combine.
Place the bowl over a saucepan of simmering water (don’t let the base of the bowl touch the water). Cook, stirring constantly, for about 10 minutes.
After 10 minutes, the curd should be thickened. It’s ready when reduced significantly and coats the back of a wooden spoon.
Add the butter to the hot curd and whisk to melt and fully incorporate.
Pour your finished lemon curd into a glass jar and loosely cover. Set aside to cool to room temperature before sealing and refrigerating.
After refrigerating and cooling fully, the curd should be set and pudding-like in texture. It will keep up to a couple weeks in the refrigerator and can be frozen up to a month.