Step 1: In a medium, heavy-bottomed saucepan, combine the lemon zest, fresh lemon juice, and granulated sugar. Heat over medium-low heat, stirring occasionally, until the sugar is completely dissolved and the mixture is warm, but not boiling.
Step 2: In a separate medium bowl, whisk together the whole eggs and egg yolks until well combined and slightly frothy.
Step 3: Slowly drizzle about 1/2 cup of the warm lemon mixture into the whisked eggs, whisking constantly to temper the eggs. This prevents them from scrambling. Once combined, pour the tempered egg mixture back into the saucepan with the remaining lemon mixture.
Step 4: Return the saucepan to medium-low heat. Cook, stirring constantly with a whisk or heat-proof spatula, making sure to scrape the bottom and sides of the pan. Continue until the mixture thickens enough to coat the back of a spoon and leaves a clear path when you run your finger across it (approximately 8-12 minutes). Do not let it boil.
Step 5: Immediately remove the saucepan from the heat. Add the cold, unsalted butter, one piece at a time, whisking vigorously until each piece is fully melted and incorporated into the lemon curd.
Step 6: For an extra silky smooth texture, pour the hot lemon curd through a fine-mesh sieve into a clean bowl. Press it through with a spatula to remove any cooked egg bits or zest.
Step 7: Place a piece of plastic wrap directly on the surface of the lemon curd to prevent a skin from forming as it cools. Allow it to cool completely at room temperature, then refrigerate for at least 2-3 hours, or preferably overnight, until fully chilled and set.
Step 8: Once chilled, your ultimate silky smooth lemon curd recipe is ready to enjoy! Store in an airtight container in the refrigerator for up to 2 weeks.