Pour 1 cup (240 ml) of the coconut milk into a small bowl and whisk in the cornstarch until no lumps remain. Set the slurry aside.

Pour the remaining 2 cups (480 ml) coconut milk into a 2-quart / 1.9L saucepan with the sugar and salt. Set over medium heat and whisk until it steams and the sugar dissolves, about 3 minutes.

Whisk the cornstarch slurry back into the pan. Cook over medium-low heat, whisking constantly, for 5 to 6 minutes, until the pudding thickens enough to coat the back of a spoon and holds a ribbon off the whisk. Don't walk away here, it can catch on the bottom of the pan fast once it starts to thicken.

Remove from the heat and stir in the vanilla extract.

Divide the pudding evenly among 4 ramekins or small cups (about 170 ml / 6 oz each). Tap each cup on the counter to knock out any air bubbles.

Press a piece of plastic wrap directly onto the surface of each pudding to stop a skin from forming, or cover the cups loosely.

Refrigerate for at least 3 hours, or until fully set, cold, and firm when you press the surface.

While the pudding chills, toast the coconut flakes in a dry 8-inch / 20 cm skillet over medium heat for 2 to 3 minutes, stirring often, until golden and fragrant.

Top each pudding cup with the toasted coconut just before serving. Add fresh mango or berries if using.