In a microwave-safe bowl, combine sugar and lemon zest. Rub the zest into the sugar with your fingers to release the oils.
⅔ cup sugar, Zest of 2 lemons
Whisk in the lemon juice and eggs until well combined. Make sure that you whisk the mixture until you don't see any more egg whites. I like using an immersion blender for this as well to make it easier.
½ cup fresh lemon juice, 4 large eggs
Microwave the mixture in 30-second intervals, stirring well after each interval, for about 4 minutes total, or until the mixture has reached 160F and has thickened completely. Add in the cold butter and mix vigorously or use the immersion blender again.
¼ cup unsalted butter
Strain the curd through a fine-mesh sieve to remove any cooked egg bits.
Transfer the curd to a jar or container, cover the top with plastic wrap making sure the plastic wrap touches the curd, and let it cool to room temperature. Then, cover and refrigerate until chilled and set, about 1-2 hours.