Preheat the oven to Gas Mark 4/180C/fan oven 160C. Lightly grease a square loose bottomed 20x7.5 cm cake tin and fully line with nonstick baking paper.
To skin the hazelnuts add 500g of boiling water and 4tbsp of baking soda into a pan. Add the hazelnuts and let it boil for a few minutes. The water will develop a foamy surface and the colour of the water will turn brown. Rub the skin of one of the hazelnuts to check if it is ready. Once they are done, remove the pan from the hob and immerse the nuts in cold water before you peel the skins off.
Finely grind the hazelnuts in a mini processor. Put the butter and sugar in a bowl and cream together with a wooden spoon. Whisk in the eggs and yogurt until fluffy. Sift the flour and baking powder together and fold in with the ground hazelnuts.
Fold in the sliced apples and spread the mixture evenly into the tin. Sift over the icing sugar. Put the tin on a baking tray and cook for 1hour.
Meanwhile prepare the topping. Put the butter, sugar and cream into a small pan and melt together. Stir in the nuts and fold in the apple.
Spread the mixture over the cake when it has cooked and put back in the oven to bake for a further 10-15 minutes.
Put the tin on a wire rack to cool. When still warm lift out of the tin and serve warm or cold.