In a medium saucepan, melt the butter over medium heat with the sugar and salt. Stir until the butter is fully melted, and the sugar is incorporated (the sugar will not completely dissolve at this point).

While the butter mixture is heating up, prepare the tempered eggs. Whisk together in a separate bowl, the finely grated lemon zest, eggs, egg yolks, fresh lemon juice, and cornstarch until well combined.

Temper the eggs: Slowly pour a small amount of the hot butter mixture into the egg mixture, whisking continuously. This will gradually raise the temperature of the eggs without curdling them.

Once the eggs are tempered, pour the entire egg mixture back into the saucepan with the remaining butter mixture. Whisk everything together thoroughly.

Continue to cook the lemon curd over medium heat, whisking constantly, until it thickens and just begins to bubble. This should take about 8 minutes. Make sure not to bring it to a full boil.

Remove the saucepan from the heat and strain the lemon curd through a sieve into a clean bowl to remove any lumps or zest.

Place a piece of plastic wrap directly on the surface of the lemon curd to prevent a skin from forming. Let it cool to room temperature.

Once cooled, refrigerate the lemon curd until completely chilled. It will set further as it cools.

The lemon curd can be stored in the refrigerator for up to 3 weeks.