Grease and line a square 25cm (10 inch) baking dish and line with baking paper. Preheat oven to 160C (315F).
Combine flour, custard powder, sugar and coconut in a bowl and stir in melted butter and 1 teaspoon of the vanilla. Press into the base of the baking dish and bake in oven for 30 minutes, or until golden. Remove from oven and set aside.
Increase the temperature to 180C (350F).
Combine in a bowl the sour cream, eggs, powdered sugar and remaining ½ teaspoon of vanilla.
Squeeze the excess juice from the apple and distribute the apple and sultanas over the base, then sprinkle over the brown sugar. Pour egg and sour cream mixture over the top and place in the oven for a further 30 minutes, or until set and turning golden.
Remove from oven to cool. The slice can be removed from the baking dish when completely cold. Serve sprinkled with toasted coconut, if desired.