To make this incredibly moist carrot cake, first up pre-heat your oven to 160C Fan, 180C, 350 F, and grease and line two 8 inch sandwich tins. Grab yourself a large mixing bowl and whisk together the sugar, oil, and vanilla. Add in the eggs, one at a time, whisking well between each addition. Using a spatula fold in the flour, bicarbonate of soda, salt, mixed spice, and cinnamon. Add in the carrot, raisins, and walnuts, mixing until well combined. Divide the mixture between your two tins and bake for 35-45 minutes. If you are not sure if the cake is, gently push a cocktail stick through the centre and if it comes out clean it is done. Leave to cool in the tins for 10 minutes, then turn on to a cooling rack to completely cool down. To make the icing place the cream cheese, mascarpone, icing sugar, and lime zest and juice into a bowl and mix together. Leave in the fridge until you are ready to use it. Place a thin layer of the icing on to the bottom sponge, then place the second sponge on top. Spread a thick layer of icing across the top of the cake, and sprinkle with the chopped walnuts. This delicious carrot cake is now ready to enjoy!