Rub the caster sugar and lemon zest together in a small bowl using your fingertips until the sugar becomes fragrant and slightly damp. This helps release the natural oils from the zest and boosts the lemon flavor.

Transfer the lemon sugar to a small saucepan. Add the egg yolks and whisk until smooth, then add the lemon juice, butter, and salt and whisk again to combine. Place the pan over medium to medium-low heat.

As the butter melts, whisk occasionally. Once fully melted, whisk continuously in a slow, steady motion. After about 3 to 4 minutes, the mixture will start to bubble gently and thicken. Continue whisking for another minute until it reaches a custard-like consistency that coats the back of a spoon.

Remove the saucepan from the heat. If you notice any small bits of cooked egg, strain the curd through a fine sieve into a clean bowl.

Cover the surface of the lemon curd directly with plastic wrap to prevent a skin from forming. Let it cool at room temperature for about 30 minutes, then refrigerate for at least 1 hour until fully chilled and set.