Preheat oven to 180°C. Grease and line a 20cm round cake tin with paper.
Beat the eggs and sugar together in a large bowl with an electric mixer until pale. Gradually add the oil. Stir through the flour, walnuts, grated carrot, mixed spice and cinnamon, being careful not to over mix. Pour into the prepared tin and bake for 50 minutes or until a skewer comes out clean. Let cool completely before icing.
To make the cream cheese icing, combine all remaining ingredients in the bowl of an electric mixer and beat on medium speed for 10 minutes until soft and fluffy.
Spread the cooled cake with the icing and garnish with chopped walnuts.
TIP: The butter and cream cheese must both be completely soft to prevent tiny lumps.