Prepare the SaucepanAdd 1-2 inches of water to the bottom of a medium saucepan. Bring the water to a boil, then reduce the heat to low, keeping it simmering.
Set Up the Double BoilerPlace a heatproof glass bowl on top of the saucepan, ensuring the bottom isn't touching the water.
Combine Base IngredientsIn the glass bowl, add sugar, lemon juice, egg yolks, and lemon zest. Whisk to combine well.
Cook and ThickenContinue cooking while whisking nearly constantly for about 8-10 minutes until the curd thickens. If it's not thickening, increase the heat to medium-low while whisking.
Incorporate ButterRemove the pan from heat and whisk in the butter pieces until melted completely.
Store and CoolTransfer the lemon curd to a bowl or smaller containers, covering the top with plastic wrap directly on the surface to prevent a skin from forming.
Chill Before ServingLet the curd come to room temperature, then refrigerate for at least 1-2 hours before serving. The curd will thicken more as it cools.
Notes
Use a microplane to zest lemons, keep whisking to avoid lumps, taste test for sweetness, cool in smaller containers for quicker results, and store in an airtight container.