Heat the oven to 180C (fan 160C).
Put the sourdough starter, sugar, sunflower oil, vanilla essence and eggs into the mixing bowl and mix well.
Add the grated carrots and orange zest and mix
Sift the self-raising flour, bicarbonate of soda and spices. Add to the mixing bowl and mix well
Grease and dust the inside sides of an 18cm square cake tin.
Pour the batter into the baking tin.
Bake for 40-45 mins. Test with skewer
Allow to stand in the tin for 5/10 minutes and allow to cool on a wire rack.
Whisk the ingredients for the frosting except for the zest and apply. Finally sprinkle the zest over the frosting.