Preheat your oven to 180C/350F.
Grease and line two 22cm round cake tins, or a large bundt tin.
In an electric mixer, beat the eggs and brown sugar until light and fluffy, about 4 minutes.
Add the sunflower oil and continue to beat until well combined.
Sift in the cake flour, salt, baking powder, and baking soda. Mix briefly to combine.
Remove from mixer and fold in the grated apple, mashed banana, and walnuts.
Divide the batter evenly between the prepared tins.
Bake for about an hour, until a sharp knife inserted in the center comes out clean.
Allow the cake to cool before transferring it to a wire rack to cool completely.
To prepare the frosting, beat together the muscovado sugar, butter, and caramel until pale and fluffy.
Add the cream cheese and beat until smooth and creamy.
Once the cakes are cool, spread frosting between the layers and cover the top and sides.
Decorate with extra walnuts.
