Warm the milk: Add the milk (and cream if using) to a medium saucepan with the salt and any optional zest or spices. Heat over medium until it’s steaming and small bubbles form around the edges. Do not let it boil.

Whisk the yolks and sweetener: In a separate bowl, whisk the egg yolks with the powdered sweetener until smooth and slightly pale. If using cornstarch or arrowroot, whisk it in now until fully dissolved.

Temper the eggs: Slowly pour about 1/2 cup of the hot milk into the yolk mixture while whisking constantly. This gently warms the eggs so they don’t scramble. Continue with another 1/2 cup, whisking well.

Combine and cook: Pour the tempered mixture back into the saucepan with the remaining milk. Cook over low to medium-low heat, stirring constantly with a heatproof spatula, making sure to sweep the bottom and corners of the pan.

Watch for thickening: The custard is ready when it coats the back of a spoon and a finger swiped across leaves a clean line, about 5–8 minutes. Avoid boiling. If you have an instant-read thermometer, aim for 170–175°F (77–80°C).

Finish with vanilla: Remove from heat and stir in the vanilla extract. If you used a vanilla bean, remove the pod.

Strain for smoothness: For a perfectly silky texture, pour the custard through a fine-mesh sieve into a clean bowl or jug. This catches any tiny bits of cooked egg or zest.

Chill or serve: Serve warm right away, or press plastic wrap directly onto the surface to prevent a skin from forming and chill for at least 2 hours. Custard will thicken more as it cools.

Optional baked custard: For a firmer set, divide the mixture among ramekins, place in a baking dish, add hot water halfway up the sides, and bake at 325°F (165°C) for 25–35 minutes until just set with a slight wobble. Cool before serving.