In a saucepan, rub the lemon zest into the sugar until fragrant and slightly moist.
Add the egg yolks. Whisk in the egg yolks until the mixture is smooth and pale.
Add the cornstarch, flour and salt, whisking well until fully combined.
Gradually pour in the cold milk, whisking constantly to keep the mixture smooth.
Mix in the lemon juice until fully incorporated. Add a pinch of turmeric if desired to enhance the color.
Place the saucepan over medium heat and cook, stirring constantly, until the mixture thickens and starts to bubble gently.
Remove from heat and stir in a tablespoon of butter for extra smoothness.
Strain if you want it extra smooth, then cover with plastic wrap touching the surface and chill.