Preheat the oven to 180C/Gas4; grease an 18cm round, loose-bottomed or springform cake tin and line with non-stick baking parchment
Beat the butter with the muscovado sugar until pale and fluffy; sieve the flour and the cinnamon into the mixture, add the beaten eggs and beat together; stir in the milk
Peel the apples, cut them into quarters, then remove and discard the cores; cut each apple quarter in half again lengthways, then slice the pieces widthways into large chunks
Reserve a handful of the apple chunks and stir the rest into the cake mixture with the pecans and sultanas; pour the mixture into the prepared tin and using the back of a spoon level the surface; scatter the reserved apple pieces evenly over the top then sprinkle over the sugar crystals; bake in the preheated oven for between 1 hour and 1 hour 10 minutes or until a skewer inserted in the centre, comes out clean; serve warm or cold
8 servings