1-First Step: Mix the base In a medium saucepan, whisk together the 4 large egg yolks, 2/3 cup granulated sugar, 1 tablespoon lemon zest, 1/3 cup fresh lemon juice, and 1/8 teaspoon salt. Whisk until the mixture looks well combined and slightly lighter in color. The sugar should start dissolving right away.

2-Second Step: Cook low and slow Place the saucepan over low heat. Cook the mixture slowly, whisking constantly, for about 8 to 10 minutes. Keep the heat gentle so the eggs do not curdle. The mixture will start to thicken as it warms. At first it will look thin and bubbly. After a few minutes, it should become smooth, glossy, and more pudding-like. A good test is to dip a spoon into the curd and run your finger across the back of it. If the line holds, the curd is ready for the butter.

3-Third Step: Watch the temperature If you use a thermometer, aim for about 170 to 175 degrees Fahrenheit. That range is hot enough to thicken the curd but not so hot that it scrambles the eggs. If you see steam rising strongly or tiny bits forming, pull the pan off the heat right away and whisk harder. Low heat is the secret to silky lemon curd. Patience here gives you a smooth finish instead of a grainy one.

4-Fourth Step: Add the butter Remove the pan from the heat. Add the 6 pieces of softened butter one at a time, whisking until each piece melts before adding the next. This step makes the curd rich, glossy, and extra smooth. If you add all the butter at once, it may take longer to melt evenly.

5-Fifth Step: Strain for the smoothest texture For an especially silky lemon curd, pour it through a fine-mesh strainer into a clean bowl. Use a spoon or spatula to push it through. This removes any tiny cooked egg bits, zest pieces, or lumps. If your curd looks perfect already, you can skip this step, but straining gives a beautifully polished texture.

6-Final Step: Cool and chill Let the lemon curd cool for about 15 minutes at room temperature. Then transfer it to a jar or airtight container. Cover the surface with plastic wrap if you want to prevent a skin from forming, then refrigerate until fully chilled. It will thicken more as it cools, usually within 1 to 2 hours.