Preheat the oven to 180ºC (350ºF, gas mark 4). Grease a 20 cm (8 in) round deep cake tin and line the bottom with baking parchment.
Sift the wholemeal and white flours and the cinnamon into a large bowl, tipping in any bran left in the sieve. Coarsely chop about two-thirds of the brazil nuts and stir into the flour with the raisins. Thinly slice the rest of the brazil nuts lengthways and set aside.
In another bowl, beat together the sugar and oil with a wooden spoon until well combined. Beat in the eggs one at a time, then stir in the grated carrots and orange zest and juice. With a large metal spoon, carefully fold the carrot mixture into the flour mixture, just until combined. Do not overmix.
Spoon the mixture into the prepared tin. Bake for 50 minutes or until risen and firm to the touch. Leave the cake in the tin for 5 minutes, then turn out onto a wire rack and peel off the lining paper. Cool completely.
To make the icing, put the ricotta in a bowl, add the sugar and orange zest, and beat with a wooden spoon.
When the cake is cold, spread the icing on top. Scatter over the reserved sliced brazil nuts, letting some stick up out of the icing at different angles. The cake can be kept, covered, in the fridge for up to 3 days.