Preheat the oven to 180 degrees Celsius (350 Fahrenheit).
In a large mixing bowl sift the flour, add the baking powder and bicarb of soda, sugar, spices, chopped walnuts and raisins and mix well.
In a jug, crack the egg in and beat it well with a fork.
Add the oil and vanilla extract and mix.
Mash the banana, grate the carrot and squeeze out the excess liquid.
Add them to the jug and mix well.
Combine the wet ingredients with the dry ingredients.
Use a spatula to mix everything into a batter.
Oil and flour a standard 2lb loaf tin, add the batter and bake for 40-45 minutes or until a skewer inserted in the middle comes out clean.
Remove the cake from the oven, and allow it to cool down completely.
To make the frosting, use a hand mixer to beat the butter and icing sugar, then add the cream cheese and vanilla extract and beat again to get a smooth texture.
Once the sponge has cooled down, run a sharp knife around the edges of the tin, it will come out easily.
Decorate with the frosting and chopped walnuts.