In a small saucepan, whisk together the eggs and egg yolks.
3 large eggs, 4 large egg yolks
Then whisk in the lemon juice, lemon zest, sugar, and cornstarch.
⅔ cup freshly squeezed lemon juice, 2 teaspoons lemon zest, 1 cup granulated sugar, 1 teaspoon cornstarch
Place the saucepan on the stove and cook over medium-low heat, whisking the mixture constantly as it heats. Keep whisking and cooking the mixture until it thickens to the point that it will coat the back of a spoon. If you have an instant-read thermometer, you want it to reach 170°F. It may take up to 10 minutes, and remember that it will continue to thicken when it is chilled. You must whisk it constantly, even while checking the temperature. I really love this rubber spatula with a built-in thermometer! Once the curd has thickened, remove from heat and stir in the butter until fully incorporated, then stir in the vanilla.
½ cup salted butter, ½ teaspoon vanilla extract
If the lemon curd is not completely smooth, you will want to strain it through a fine mesh sieve.
Allow the curd to cool for about 15 minutes. Then place a piece of plastic wrap directly on the surface of the curd (so it doesn’t develop a film) and place this into the fridge. It should chill for at least 3 hours before using as desired.