In a small to medium size mixing bowl, whisk together the sugar and eggs until smooth. Add the lemon juice, melted butter and lemon zest (if desired). Stir to combine.

Microwave the lemon curd mix in 30 second to 1 minute increments (I like to microwave the lemon curd in shorter increments, but it will take a bit longer to get your desired consistency). Stir between each microwave heating.

Repeat the heating and stirring steps until your lemon curd sticks to the back of your spoon and begins to hold the stirred shape in your bowl (it will set up more when cooled and refrigerated). This step can take between 4-10 minutes, depending on your microwave's power. When done, the lemon curd should have an internal temperature of 185 degrees F (85 degrees C).

Stir and transfer to a storage container, then refrigerate until your lemon curd is firm. Lemon curd will store, refrigerated, for up to three weeks and can be frozen for longer storage.