Add 1-2" of water to the bottom of a medium saucepan. Bring water to a boil, then reduce heat to low and keep water simmering.
Place a glass (and heat-proof) bowl on top of the saucepan, so the bottom isn't touching the water.
To the glass bowl, add sugar, lemon juice, egg yolks, and lemon zest. Whisk to combine well.
Continue cooking, and whisking nearly constantly for about 8-10 minutes. Curd should be thickening.
If your lemon curd isn't thickening at all, increase the heat to medium low and continue whisking and cooking.
Remove pan from heat, then add butter and whisk to melt completely.
Transfer lemon curd to a large bowl, or smaller containers. Place a piece of plastic wrap directly on top of the curd, pressing very gently so the entire top surface is covered. This prevents a "skin" from forming on the curd.
Let curd come to room temperature, then place in the refrigerator for at least 1-2 hours. Once cooled, the plastic wrap can be removed.
Curd will thicken more as it cools and chills.
Lemon curd will keep in the refrigerator for 1-2 weeks, or can be frozen for up to 3 months.