1. Zest and juice the lemons until you have a half-cup of juice. You can add the zest with the juice but you will want to strain it out at the end. (Or save the zest for another use, like making citrus sugar or citrus salt!)
2. In a heavy-bottomed medium saucepan, whisk together the egg yolks and sugar until well-combined. Add the lemon juice to the pan and stir to combine.
3. Gently heat the mixture over medium-low to low heat, stirring constantly until thick enough to coat the back of a spoon, about 10 to 15 minutes. Do not let curd boil!
4. When curd has thickened, remove from heat and stir in the butter. (The residual heat of the curd will melt the butter.)
5. Strain the curd through a fine-mesh strainer into a bowl to remove any pieces of cooked egg. Pour curd into glass jars.
6. Refrigerate until needed. Lemon curd will keep for several weeks.

Note: Lemon curd can be frozen as well. Leave extra room at the top of the jars for expansion.