In a large microwave-safe bowl, whisk together the sugar, eggs, egg yolks, lemon juice and salt (⅛ teaspoon if using unsalted butter) until smooth. Add butter cubes. Microwave on high power for 45 seconds. Remove and stir vigorously with a whisk.
Continue to cook 45-second intervals, stirring after each interval. It will take 4-7 minutes, depending on the power of your microwave. Lemon curd is done cooking when it coats the back of a metal spoon. You can also check it with an instant thermometer. It’s done when it reaches 175˚F.
See Café Tips above for further instructions and more detailed tips.
If using unsalted butter add a pinch (about ⅛ teaspoon of salt).