Add the eggs, lemon zest, lemon juice, sugar, and salt to a large microwave-safe bowl. Whisk smooth then let the mixture sit for 10 minutes for the sugar to dissolve.
After 10 minutes, vigorously whisk the mixture. 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 it.
If you are left with bits of scrambled egg in your curd after cooking, push the curd through a mesh strainer using the back of a soup spoon. Discard any solids left behind.
Don't be alarmed if your lemon curd goes slightly over the 170ºF (76ºC) temperature point. It must reach at least 170ºF (76ºC) to reach the correct thickness and setting point.
Storage
Microwave lemon curd can be stored in the fridge for up to one week in glass jars or containers with tight-fitting lids.
Lemon curd freezes well. Place the curd in an airtight freezer-safe container for up to 2 months. To use, thaw overnight in the refrigerator before needed.