Nutty apple streusel cake - Good Food Middle East

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)