Mix the sultanas and dried apples with five tablespoons of Calvados and leave to soak overnight.
For the cake, pre-heat the oven to 180C/350F/Gas 4 and grease a 23cm/9in springform tin. Toast the nuts in a hot dry pan until they are golden brown. Transfer to a food processor, pulse until chopped and set aside.
For the fruit topping, melt the sugar in heavy-based saucepan to form a golden caramel. Remove from the heat and add the butter, stirring gently until incorporated into the caramel (take care not to splash yourself as caramel is very hot). Place the pre-prepared tin on a baking sheet and pour in the caramel. Carefully arrange the apple slices on top until the base of the tin is completely covered.
For the cake, beat together the flour, margarine, sugar, eggs, vanilla, baking powder and cinnamon until combined and airy. Fold in the chopped hazelnuts and soaked fruit before pouring onto the apples and gently smoothing the surface.
Bake in the oven for 40-50 minutes or until light golden-brown and springy to the touch. Set aside to cool in the tin.
For the praline, fill a large bowl with cold water. In a heavy-based pan melt the sugar to a golden-brown caramel, swirling the pan occasionally so it doesn’t burn and using a damp pastry brush to brush down any sugar crystals from around the edge of the pan. Remove from the heat and dunk the base of the pan in the cold water to stop the caramel cooking.
Place a heavy chopping board on the edge of the work surface and spread newspaper on the floor beneath. Spear half the hazelnuts onto cocktail sticks. Carefully dip each nut in the caramel before securing the free end of the cocktail stick under the chopping board, so the nut overhangs the work surface allowing a spike of caramel to form and harden (the paper will catch any drips).
Gently reheat the remaining caramel until it becomes runny again. Add the remaining hazelnuts, swirl to coat, and pour onto a lined baking sheet. Set aside in a cool dry place. When completely set, blend in a food processor to make a praline.
When the cake is cool, prick with a cocktail stick and spoon over the Calvados. Leave in the tin for 5-10 minutes to allow the Calvados to infuse the cake.
Run a sharp knife around the edge of the cake and place a serving plate over the top. Quickly flip the cake cover, remove the tin and rearrange any apples that may have come away.
To serve, sprinkle with praline and decorate with the spiky hazelnuts.