To make the lemon curd, add the lemon juice, large egg yolks, granulated white sugar, salt and lemon zest to a medium saucepan or pot. Whisk everything together.
½ cup (120 ml) fresh lemon juice, 4 large egg yolks, at room temperature, ¾ cup (150 g) granulated white sugar, ⅛ teaspoon salt, 1 tablespoon (10 g) lemon zest
Heat the lemon mixture over medium-low heat for 20-25 minutes, whisking frequently, until thick. It should coat the back of a spoon once thick enough.
Remove it from the heat and slowly whisk in the cold, cubbed butter.
½ cup (112 g) unsalted butter, cold and cut in cubes
Once all of the butter is melted in, transfer the curd to a heatproof, small bowl. Place plastic wrap on top of the curd, so it doesn't form a skin, and allow it to cool completely in the fridge.
Store leftovers in an airtight glass jar in the fridge for up to two weeks.