Preheat the oven to 170C (375F/Gas Mark 5). Grease a deep 23cm loose-bottomed cake tin and line with baking parchment.
Mix together the sultanas and whiskey together in a small bowl and set aside to soak while you make the cake.
Cream the butter and sugar together in a large bowl, food processor or free-standing mixer until light and fluffy. Gradually add the eggs, mixing well after each addition. Mix in the vanilla paste or extract.
Sift the flour and baking powder into the batter and pulse or lightly mix until combined. Then add the chopped apple, sultanas and whiskey and combine until just mixed. Spoon the mixture into the prepared cake tin and smooth the top.
Lay the sliced apples on top of the cake, pushing them in slightly. Sprinkle with the caster sugar and then the demerara sugar.
Bake on the bottom shelf of the preheated oven for 60-75 minutes or until a skewer inserted into the centre comes out clean. Keep an eye on the cake and if you need to turn it around in the oven to ensure even colouring then do so.
Remove from the oven and allow to cool in the tin for 20mins before transferring to a wire rack to cool further.
Dust with icing sugar before serving.