Add the eggs, lemon zest, lemon juice, sugar, and salt to a large microwave-safe bowl. Use a hand mixer or handheld whisk to mix everything together until smooth. Let the mixture sit for 10 minutes for the sugar to completely dissolve.
After the 10 minutes are up, give the mixture a vigorous whisk then break the butter into small pieces and add them to the bowl.
Microwave the mixture for one minute on full power. Remove the bowl from the microwave and take a temperature reading. The goal is to reach 170ºF (76ºC).
Whisk the mixture briskly again to help melt the butter pieces and make the curd smooth. Return the bowl to the microwave and cook for one minute.
Remove the bowl from the microwave and take a temperature reading. After each minute of cooking the temperature will climb. Whisk the lemon curd again before returning it to the microwave. When your microwave lemon curd reaches around 150ºF (65ºC), reduce the cooking time to 30 second increments. When the curd reaches 170ºF (76ºC) it's done!
Give the curd one good final whisk. At this point it should be thick, like the consistency of honey.
Spoon the lemon curd into clean jars or containers. Leave the lids of the jars slightly ajar for a few hours to prevent condensation from dripping into your curd. Let the curd cool completely before storing in the fridge with the lid secured tightly.
Notes
If you don't like lemon zest simply omit.You must whisk the curd vigorously in between each increment of cooking. Hard whisking helps eliminate the risk of scrambled egg bits in your microwave lemon curd. I prefer to use an electric hand mixer for this reason.Don't be alarmed if your lemon curd goes over the 170ºF (76ºC) temperature mark. The last batch I made reached 178ºF (81ºC) and was still perfect! It must get to at least 170ºF (76ºC) in order to reach the correct thickness and setting point. In and around that point is fine, just don't let the temperature climb too high at risk of curdling and/or scrambling.Microwavelemon curd will keep, stored in glass jars or containers with tight-fitting lids, for up to one week.
SUBSTITUTIONS
Make this recipe with any other citrus you like.I've only made this recipe with white sugar, however, my friend Alex has a recipe for healthier lemon curd made with honey.Butter adds silkiness and richness to lemon curd, however, you can make dairy-free lemon curd by swapping in a high quality dairy-free butter.