Heat oven to 400 F.In a 9" cake pan , press shortbread crust evenly, prick all over with a fork, place foil on top, and weigh down with beads or pie weights. Bake "blind" ie. no filling, for 20 minutes. Remove from oven, allow to cool.
Spread jam over the pastry base. Sprinkle half the coconut over the jam.
Bring milk and sugar to a boil, remove and stir in custard powder, whisk until thick. Spread over the jam. sprinkle with the rest of the coconut. Let cool and serve.