Preheat your oven to 180°C (Fan 160°C) or Gas Mark 4. Grease and line a 9-inch (23 cm) deep round cake tin with baking parchment.
In a large bowl, measure the butter, sugar, eggs, chopped walnuts, sultanas, flour, and baking powder. Beat well for about 2 minutes until thoroughly blended.
Spoon half the mixture into the prepared tin, spreading it evenly.
Layer the grated apple and sprinkle the ground cinnamon evenly over the mixture.
Spread the remaining cake mixture on top and level the surface. Sprinkle generously with light muscovado sugar and chopped walnuts.
Bake in the preheated oven for 1 ¼ to 1 ½ hours or until well risen and golden brown.
Cool the cake in the tin for a few minutes, then transfer it to a wire rack. Dust with icing sugar before serving.
