In a medium saucepan, combine the eggs, sugar, salt, lemon juice, and lemon zest. Make sure all ingredients are ready and measured for a smooth cooking process.
Whisk the mixture thoroughly over low heat. Stir constantly, ensuring there are no lumps, until the mixture is smooth and well combined.
Continue stirring the mixture over low heat. Watch for it to thicken, which should take about 4–5 minutes. Be patient and keep the heat low to prevent curdling.
Once the mixture has thickened, turn the heat to a lower setting and add in the diced butter. Stir until the butter has fully melted and the mixture is creamy.
Remove from heat and strain the curd into storage jars to remove any zest pieces or lumps. Once cooled, store in the refrigerator for up to a week or freeze for extended storage.
Notes
Use fresh lemons for the best flavor, monitor heat closely to avoid curdling, and strain for a smooth texture.