Preheat the oven to 180°C and grease and line a 20cm round cake tin.
Break the eggs into a bowl and whisk lightly using a fork.
Add the vegetable oil to the eggs and whisk again to combine.
Stir in the grated carrots, raisins, walnut pieces, and orange zest.
Sift the flour, mixed spice, bicarbonate of soda, and salt into a separate bowl and stir in the sugar.
Merge the wet carrot mixture with the dry ingredients and mix well.
Spoon the batter into the tin and bake for 60 to 75 minutes until golden-brown.
Cool the cake in the tin for 15 minutes before transferring to a wire rack.
Beat the butter and caster sugar for 3 minutes before blending in the cream cheese.
Spread the frosting over the cooled cake and decorate with extra walnuts.