Pre-heat your oven to 180°c (160°c for fan-assited ovens or Gas Mark 4). Grease and line two 7 inch sandwich cake tins with grease-proof paper. Set aside.
In a large bowl, mix the oil, caster sugar and brown sugar, before adding in the eggs; one at a time ensuring they are mixed through well. Add the carrot, raisins/sultanas, walnuts & orange zest and mix to combine.
Sift in the flour and bicarbonate of soda, add the cinnamon and nutmeg. before folding the mixture together to create a batter.
Spoon the batter evenly into your prepared tins and bake in your pre-heated oven for 30-40 minutes.
Remove from the oven and allow to cool in the tins for a little while, before transfering to a wire rack to cool completely.
For the Buttercream:
In a large bowl, mix the butter/margarine with the icing sugar (and orange zest/cinnamon, if using), until you have a light and fluffy buttercream.
Once completely cool, spread half the buttercream on the top of one of your Carrot Cake layers, sandwich the other on top and spread the remaining buttercream on top. Finish with chopped walnuts to finish, if desired.