In a medium bowl, combine lemon zest and sugar. Rub together with your fingers until fragrant.

Put the lemon zest and sugar mixture, lemon juice, egg yolks, and butter in a medium heavy-bottomed saucepan and stir until combined. Cook over medium-low heat, stirring constantly, until the butter melts and the mixture thickens, about 12-15 minutes. The lemon curd will continue to thicken as it cools.

Remove the pan from the heat and pour lemon curd into a jar or glass bowl. Press a piece of plastic wrap directly against the curd so it is airtight. Cool to room temperature and then place in the refrigerator. Again, the curd will thicken up after it is completely cooled.

Note-Lemon curd will keep in the refrigerator for up to 2 weeks.