Nutty apple streusel cake

Name: Nutty apple streusel cake
Cook time: 30 min
Total time: 30 min
Cuisine: British
Ingredients: 200g light muscovado sugar; 200g softened butter; 4 eggs , beaten; 100g self-raising flour; 100g plain wholemeal flour; 2 tsp baking powder; 3 tbsp milk; 2 tsp ground mixed spice; 4 peeled and cored small eating apples (Cox's are ideal); 1 tbsp light muscovado sugar; 1 tbsp plain flour; 2 tbsp butter; 50g roughly chopped blanched hazelnuts; 100g butter , softened; 140g icing sugar , sifted; drop vanilla extract (optional)