Put the oil, eggs and sugar in a mixing bowl and whisk together until smooth.
Add the flour, baking powder, orange rind and ground cinnamon and whisk again until smooth. Stir in the grated carrots and chopped nuts. Divide the mixture between 2 x 20 cm (8 inch) sandwich tins, greased and base-lined with oiled greaseproof paper, and level.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 20 minutes until the tops spring back when pressed. Cool for 5 minutes, then turn out on to a wire rack and peel off the lining paper. Leave to cool.
Make the maple frosting. Pour the maple syrup into a saucepan and heat to 115°C (240°F) on a sugar thermometer. As the temperature begins to rise, whisk the egg whites and salt in a clean bowl until stiff. When the syrup is ready, whisk it into the egg whites in a thin trickle until the frosting is like a meringue mixture. Keep whisking for a few minutes more until very thick.
Cut each cake in half then sandwich 4 layers together with frosting. Transfer to a serving plate, then swirl the rest of the frosting over the top and sides of the cake. Decorate the top with the walnut pieces.