In a medium-sized saucepan, add the first 4 ingredients: lemon juice, granulated sugar, egg yolks, and lemon zest. Whisk until combined. Turn the heat up to medium-high heat.
Allow the mixture to come to a boil, and then turn the heat down to a simmer (medium heat)
Whisk constantly for 5 minutes while the mixture simmers. Once it has noticeably thickened, add the butter and stir until combined.
Pour the curd into a glass jar and cool for a bit.
Once cooled down, add the lid to the jar and place it in the fridge to completely chill. This will take a minimum of 4 hours.
You can leave the curd in your fridge for up to 2 weeks, use to top all your favorite desserts and sweet breakfast foods.