Preheat the oven to 180°C/ gas 4. Grease and line a 23cm round spring-clip tin. For the topping, rub together the cold butter and plain flour with a pinch of salt and mix in the demerara sugar until it resembles large breadcrumbs. Mix in the chopped almonds and leave to chill.
Melt 25g of butter in a frying pan and then add the apples. Sprinkle with the cinnamon and 25g caster sugar and cook over a medium heat for 4-5 minutes, until tender and caramelised. Leave aside. Mix together the softened butter and the rest of the sugar and then gradually add the eggs and 2 tbsp self-raising flour to prevent it curdling. Fold in the rest of the flour, groundalmonds and vanilla extract gradually until fully combined.
Spoon about 2/3 of the mix into the tin, add the apple slices and top with the rest of the cake mixture. Scatter the chilled crumble mix over the top.
Bake for 50 minutes, until a skewer inserted in the middle comes out clean. Leave to cool in the tin for 10-15 minutes before transferring to a wire rack. Serve with whipped cream.