Preheat the oven to 180°C/160°C/350°F/Gas mark 4. Sift the flours and custard powder together into a bowl and set aside. Beat the butter, sugar and egg until light and fluffy. Fold in the sifted flours and custard powder until fully combined.
Spread the dough evenly into the prepared baking tin. (The dough does look sparse but it will fully coat the base completely.) Spread the jam carefully over dough base.
Make the coconut topping by incorporating all the topping ingredients together then sprinkle the topping over the jam. Place into the preheated oven on a middle shelf to bake for approximately 40 minutes or until fully baked.
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