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Heat the oven to 180C/160C fan/gas 4. Line a 1.2 litre loaf tin with baking parchment so that it hangs over the edges of the long sides enough to help you lift the cake out. Rough chop the cobnuts and mix them with the apple.
Beat the butter with the sugar until it is pale and soft then beat in the eggs one by one. Stir in a little flour if the mixture looks as if it is separating. Fold in the remaining flour and baking powder followed by the apple and nut mixture.
Bake the cake for 1 hour or until it is risen and golden and a skewer comes out clean (be careful not to mistake a bit of soft apple for uncooked cake).
While the cake is hot and still in the tin melt the dulche de leche or caramel in a pan and let it bubble for a minute or two, it will darken a little. Stir in the nuts and then spoon the mixture over the top of the cake. Once the topping and cake have cooled for 5 minutes lift the cake out of the tin and finish cooling it on a rack.