Preheat the oven to 375°F. Butter and flour one 6-inch, one 8-inch, and one 10-inch cake pan, then line each with parchment paper. Sift together the cake flour, baking powder, baking soda, cinnamon, and salt. Set aside.

In a large mixing bowl, beat the sugar, oil, and eggs until light and fluffy. Stir in the grated carrots. Add the sifted dry ingredients and stir together by hand, adding the raisins last.

Pour into the prepared pans and place in the oven. Reduce oven temperature to 350°F. Bake for approximately 30 minutes, or until the cakes test done (they should still be slightly moist in the center). Let cool on wire racks before removing from the pans.

TO MAKE THE ICING, combine all ingredients in the bowl of an electric mixer and beat at low speed for 3 minutes. Scrape down the sides of the bowl and beat at high speed for 5 minutes.

Spread the icing smoothly over the top and sides of the layers, and press finely chopped walnuts into the sides. Stack the layers.