In a heavy, nonreactive 2 qt saucepan, heat the milk to a simmer. While the milk is heating, whisk the eggs and sugar together with the salt. Reduce heat to very low, whisk about half of the hot milk into the egg mixture, then dump the egg, milk and sugar mixture into the pot with the remainder of the milk. Add ⅓ cup coconut.
Cook over very low heat, stirring constantly. You don't want the eggs to "curdle" which basically means scramble, but very low heat and constant stirring should help. After about 2 minutes, combine some of the custard mixture with the cornstarch and stir together until no lumps remain. Add to the rest of the custard mixture, and keep cooking and stirring until it stops being milk and turns into custard, or until it coats the back of the spoon. You'll know. It's supposed to be still runny. Add the remaining coconut and the vanilla and stir together. Chill in the refrigerator.
Assemble the Trifle:
Slice the pound cake and set it on a rimmed cookie sheet. Sprinkle the rum over the poundcake slices (they should be damp but not soaked. Use more or less rum as you feel like it.
Peel and slice the mango. (I realize this is impossible to do neatly. Random sizes are fine here, but I'd go with slices over chunks for layering purposes).
In a tupperware container (or a bowl), layer the rum soaked pound cake, the mango slices, and the custard, finishing with the custard.
Chill for at least 2 hours before serving.
Recipe by The Domestic Front at https://www.thedomesticfront.com/mango-rum-trifle-with-coconut-custard/