Heat the oven to 160°C (320°F) and line a 8 × 12 inch cake or swiss roll tin.
Slice and peel the apple, then place the pieces in a pan with enough water to prevent burning. Heat gently.
For the cake, beat together the butter and sugar. Add the egg with a little flour and beat again, then add the remaining flour and milk and beat to incorporate. Mix in the well soften apples and turn out into tin.
Dot with raspberries then sprinkle with topping.
Cook for 45 minutes and serve however you want (as long as it’s with custard).