Beat eggs with sugar till light and fluffy. Add sifted flours with baking powder. When all is well combined, pour mixture into a large (36x25cm) baking tin lined with greaseproof paper.
Bake for 25 minutes in a preheated oven. Set aside to cool.
Peel and finely grate apples. Place in a coriander, add jelly powder and mix well. (Optionally, add cinnamon to taste.)
Bring 500ml milk to a simmer. Stir remaining 100ml milk with custard power till smooth and lump-free. Slowly add custard mixture to the simmering milk while whisking vigorously. Bring to the boil, whisking, till custard thickens. Set aside to cool.
Cream 200g margarine till white and fluffy. Whilst stirring, add cooled custard a spoonful at a time, till well incorporated.
Prepare coconut topping: Melt 100g margarine in a large frying pan. Add sugar and coconut. Brown slightly, stirring constantly. Set aside.
To assemble the cake, spread apple mixture over cooled sponge cake base. Top with custard and sprinkle generously with slightly warm toasted coconut.