Start by gathering all your ingredients. Make sure your lemons are fresh for the best flavor. Zest the lemon and then juice it, measuring out the necessary amount.
In a medium saucepan, combine the 3 large eggs and 1 cup (150g) granulated sugar. Whisk together until the mixture is smooth and pale in color.
Next, add the 1/2 cup (119ml) fresh lemon juice and a pinch of salt to the egg mixture. Whisk again until well combined.
Place the saucepan over medium heat. Cook the mixture, stirring constantly with a whisk, until it thickens and coats the back of a spoon. This usually takes about 10 minutes. Be careful not to let it boil.
Once thickened, remove the pan from heat and pour the lemon curd through a fine mesh strainer into a heatproof container. This step ensures a silky-smooth texture by removing any cooked egg bits.
While the curd is still warm, add the 4 tablespoons (57g) unsalted butter and the zest of 1 large lemon. Stir until the butter is fully melted and incorporated.
Let the lemon curd cool to room temperature, then cover it with plastic wrap, pressing it directly against the surface to prevent a skin from forming. Refrigerate for at least 2 hours before using.