Step 1: Prepare your double boiler. Fill a saucepan with about 1-2 inches of water and bring it to a gentle simmer over medium-low heat. You will place a heatproof bowl on top of this, ensuring the bottom of the bowl does not touch the water.
Step 2: In the heatproof bowl, whisk together the egg yolks, whole eggs, granulated sugar, and lemon zest until well combined and slightly pale.
Step 3: Add the fresh lemon juice and a pinch of salt to the egg mixture, whisking gently to incorporate.
Step 4: Place the bowl over the simmering saucepan. Continuously whisk the mixture. This step is crucial for preventing the eggs from scrambling and ensuring a smooth lemon curd.
Step 5: Continue whisking for 10-15 minutes, or until the curd thickens significantly and coats the back of a spoon. It should reach a temperature of 170-175°F (77-79°C). The whisk will leave trails, and the curd will hold its shape slightly when lifted.
Step 6: Remove the bowl from the heat. Immediately add the cold butter pieces, one at a time, whisking until each piece is fully melted and incorporated. This adds richness and shine.
Step 7: For an extra silky-smooth lemon curd, strain the curd through a fine-mesh sieve into a clean jar or bowl. This removes any cooked egg bits or lemon zest, though the zest can be left in for more intense lemon flavor if desired.
Step 8: Cover the surface of the lemon curd directly with plastic wrap to prevent a skin from forming.
Step 9: Chill the lemon curd in the refrigerator for at least 2-4 hours, or until completely cold and set. It will thicken considerably as it cools.