Prepare the Ingredients. Zest your lemons. You’ll need about two teaspoons of zest for this recipe. After zesting, juice the lemons until you have around 1/2 cup of fresh lemon juice. Strain the juice to remove any seeds or pulp. In a separate bowl, whisk together the eggs and egg yolks. You’ll want to mix them until they are smooth and well combined.
Cook the Lemon Curd Base. In a medium saucepan over medium heat, combine the lemon juice, lemon zest, sugar, and a pinch of salt. Stir occasionally until the sugar has dissolved completely. Once the sugar has dissolved, add the egg mixture slowly, whisking constantly to prevent the eggs from curdling. Continue cooking the mixture over medium heat, constantly stirring until it thickens for 5-7 minutes.
Add Butter and Finish Cooking. Once the mixture has thickened to a custard-like consistency, remove the pan from the heat and stir in the butter. Stir until the butter has completely melted and incorporated into the curd. If you’re using cornstarch, dissolve a teaspoon of cornstarch in a tablespoon of water and whisk it into the lemon mixture.
Strain and Cool. For an extra-smooth curd, strain the mixture through a fine-mesh sieve to remove any lemon zest and any curdled egg bits. This step ensures that your lemon curd is velvety smooth. Transfer the strained curd into a clean bowl and cover it with plastic wrap. Let the curd cool at room temperature for about 30 minutes, and then refrigerate it for at least 1 hour before serving.