Heat the Lemon Mixture: In a small saucepan over medium heat, combine the lemon zest, lemon juice, sugar, and salt. Cook for about 5 minutes, stirring occasionally, until the sugar and salt dissolve.

Temper the Eggs: In a separate medium bowl, whisk together the eggs and egg yolks until well combined. Slowly pour some of the heated lemon mixture into the eggs, whisking constantly, to temper them. (About half the liquid should be added gradually.)

Cook Until Thickened: Return the pot to the stove over medium heat and slowly stir in the tempered eggs, whisking continuously to prevent curdling. Cook for about 5 minutes, stirring constantly, until the mixture thickens. You’ll know it’s ready when you can drag a whisk through it and see visible marks.

Strain the Curd: Pour the thickened lemon curd through a fine-mesh strainer into a clean bowl to remove any solid egg bits.

Incorporate the Butter: Stir in the chopped butter until completely melted and combined.

Chill: Press a piece of plastic wrap directly onto the surface of the curd to prevent a skin from forming. Refrigerate for 2-4 hours, until fully chilled and thickened.

Store & Serve: Transfer to an airtight container and keep refrigerated for 1-2 weeks. Serve on toast, scones, pancakes, or as a filling for desserts.