Preheat the oven to 180°C/360°F/Gas Mark 4 and grease and line a loose bottomed cake tin.
Put the butter and Tate & Lyle Caster Sugar in a mixing bowl and cream together with a wooden spoon. Whisk in the eggs and yoghurt, then sift in the flour and baking powder. Fold, then add the ground hazelnuts.
Now add the apples and spread the mixture evenly into the prepared tin. Sift over the icing sugar and bake in the oven for an hour.
To make the topping, melt the butter, Tate & Lyle Light Soft Brown Sugar and cream in a small pan. Stir in the hazelnuts and the apple slices.
After an hour, remove the cake from the oven, cover with the topping, then put back in the oven to bake for a further 10 to 15 minutes.
Allow to cool a little in the tin, then lift out of the tin, sprinkle with cinnamon and serve warm or cold.
To help prevent the apple from sinking in your sponge, ensure the slices are diced into small chunks or cut finely.