Chocolate, Hazelnut & Cinnamon Cake

Preheat oven to 170u0b0C (350u0b0F) and grease and/or line a 20cm x 20cm square cake tin.
You can make this in a food processor, but as long as your butter or margarine is soft, a bowl and a wooden spoon or fork work fine (less things to wash up). Put the butter or margarine in a mixing bowl (nuke in the microwave if not already very soft) and add brown sugar. Beat with a wooden spoon or spatula until well combined. Add eggs one at a time, whisking well between each addition. Sift in flour, cinnamon, baking powder and cocoa and mix well until smooth. Finally, stir in yogurt or milk.
Put about half of the mix into your cake tin and spread evenly over the base of the tin. Dollop the chocolate hazelnut spread over the mix, fairly evenly spaced apart (it will spread, so don't get too technical, but try to keep it away from the edges).
Using a spoon, dollop the rest of the mixture on top and spread it very gently with a knife, covering the chocolate hazelnut spread, as evenly as you can. Again, don't get too hung up on it, this is not a great work of patisserie. Sprinkle the chopped hazelnuts over the top and finally, dust with brown sugar.
Bake for about 1 hour or until the centre of the cake feels firm to the touch. If it is still wobbly in the middle but getting too brown on the top, just cover with foil and then give it another 5-10 minutes. (Mine was ready after 1 hour.)