Begin by whisking the egg yolks, sugar, cornstarch, and salt together in a medium saucepan. This step is crucial because it ensures the cornstarch is evenly distributed, which prevents lumps later on. Once the mixture is smooth and well-combined, gradually whisk in the milk. Stir slowly to avoid incorporating too much air.

Place the saucepan over medium heat and cook the mixture, stirring constantly. It’s very important to stir the custard continuously so it doesn’t scorch or scramble the eggs. After about 5 to 8 minutes, the mixture will begin to thicken. You’ll know it’s ready when it coats the back of a spoon and a line drawn through it stays clean.

Once thickened, remove the pan from the heat and immediately stir in the lemon juice, lemon zest, and butter. Continue stirring until the butter is fully melted and the mixture is smooth and glossy. The heat from the custard will help the lemon juice blend evenly without curdling.

Pour the finished custard into serving dishes or a larger container, depending on your presentation. Cover with plastic wrap pressed directly onto the surface to prevent a skin from forming. Chill for at least 2 hours, or until fully set. Serve as-is or garnish with whipped cream and lemon zest for an extra-special finish.