Whisk egg yolks, lemon juice, sugar, and zest in a medium sized sauce pan.
It's important to start with low heat to gently warm the mixture and prevent the eggs from curdling or scrambling. Stir continuously using a whisk as it heats. This helps distribute the heat evenly and prevents any lumps from forming.
After a few minutes of stirring over low heat, gradually increase the heat to medium-low. This allows the curd to heat up further without overheating too quickly.
Observe and adjust heat: Pay close attention to the curd as it heats. You're aiming for a mild simmer, which means small bubbles should begin to appear on the surface of the mixture. If the heat is too high and the curd starts to bubble vigorously, reduce the heat to maintain a gentle simmer.
Cook until thickened: As you simmer and stir the curd, it will gradually thicken. The cooking time can vary, but typically, it takes around 5-8 minutes for the curd to reach the desired consistency. You'll notice that the mixture becomes velvety and coats the back of a spoon.
Turn the heat off and mix in small pieces of the butter at a time, allowing them to melt and combining them thoroughly.
Then, strain the lemon curd through a mesh strainer so the finished curd has a smooth texture.
Allow the lemon curd to cool, then store in a covered container. Glass canning jars are ideal for storage. Lemon curd can typically be stored for up to 1 to 2 weeks in the refrigerator.