Use a 20cm round, deep cake tin and line the base with baking paper. Grease lightly. Preheat oven to 180 degrees C.
Sift the flours and cinnamon into a large bowl, tipping in any bran left in the sieve. Coarsely chop about two-thirds of the brazil nuts; stir into flour with the raisins. Slice remaining nuts thinly, lengthwise; set aside.
In another bowl, beat sugar and oil with a wooden spoon until well combined. Beat in the eggs one at a time. Stir in the grated carrots and orange zest and juice. Using a large metal spoon, fold the carrot mixture into the flour mixture, just until combined. Do not overmix.
Spoon mixture into 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 baking paper. Leave to cool.
To make the icing, beat ricotta, sugar and orange zest in a bowl with a wooden spoon. Spread icing on top of cold cake; top with the reserved sliced nuts, placing them so that they stick up at different angles. Cake will keep, covered, in the refrigerator for up to 3 days.