In a pot, add whole milk, cornstarch (US) / cornflour (UK), white granulated sugar and lemon zest.
Optional: Add yellow coloring if using
Place the saucepan over medium low heat.
Within 3-4 minutes you will notice, as you whisk, the mixture becomes thicker. Ensure that you whisk constantly to prevent the custard burning or sticking to the bottom of the pot.
At this point, remove the pot from the stove but continue to whisk for at least another minute until the mixture is thick and creamy.
The custard should be done when it has thickened to a creamy consistency.
Keep in mind that the mixture can continue to thicken a bit even after being removed from heat.
To check if the custard is ready: Just dip a spoon into it, making sure it coats the back of the spoon. Then, swipe your finger across the back of the spoon – the custard should hold its shape and not immediately fill in the gap.
Strain if Necessary: If you notice any lumps or bits of zest in the custard, you can strain it through a fine mesh strainer to achieve a smoother texture.
Serve or Chill: Once the custard has cooled slightly, you can either serve it warm or transfer it to individual serving dishes.
If you prefer your custard chilled, cover it with plastic wrap, ensuring the wrap touches the surface of the custard to prevent a skin from forming. Refrigerate until thoroughly chilled.