Preheat the oven to 180C/160C Fan/Gas 4. Oil and line 2 x 20cm/8in sandwich tins.
Pour the oil into a large mixing bowl with the eggs and agave syrup and beat together. Add the carrots and mix together.
In a separate bowl, sieve the flour, baking powder, cinnamon and mixed spice together. Add the sultanas, walnuts and orange zest and stir to coat in the flour.
Tip the flour mixture into the carrot mixture and stir together gently, until well combined. Divide the mixture between the tins and bake for 40 minutes. Cover with aluminium foil after 20-30 minutes to prevent the tops from burning.
Leave to cool in the tins for 5 minutes, then transfer to a wire rack and leave to cool completely.
For the frosting, place the cream cheese in a piece of clean muslin or thin cleaning cloth (such as a J Cloth) and squeeze out any liquid. (This removes the moisture from the cheese so that the frosting isn’t too wet). Place the cream cheese in a bowl and beat until smooth. Add the butter and maple syrup and beat together. Refrigerate until needed.
Sandwich the sponges together with half of the frosting. Spread a very thin layer of frosting over the top of the cake and spoon the remaining frosting into a piping bag fitted with a 1cm/½in plain nozzle. Sprinkle the chopped walnuts over the top of the frosting.
Pipe small kisses of frosting around the edge of the cake and dust with cinnamon.