Everyone seems to have an opinion on carrot cake, especially in our test kitchen. In our (lengthy) discussions, we debated the merits of adding common ingredients like coconut, pineapple, and chopped nuts. We even discussed just how much carrot to add. In the end, we all agreed that the classic carrot should definitely be loaded with carrots, walnuts, and raisins. One thing the test kitchen could agree on was that carrot cake must be frosted with cream cheese frosting. That is NOT negotiable.
Make the cake: Preheat the oven to 350˚ F. Coat three 8-inch round cake pans with cooking spray and line the bottoms with parchment paper; coat the parchment with more cooking spray. Spread the walnuts on a baking sheet and bake until toasted, 8 to 10 minutes. Let cool, then roughly chop 1 1/2 cups walnuts and finely chop 1 1/2 cups. Set aside the finely chopped nuts for topping.
Whisk the flour, baking powder, baking soda, salt, cinnamon, ginger and nutmeg in a medium bowl. In a large bowl, beat the vegetable oil, granulated sugar and brown sugar with a mixer on medium-high speed until smooth, about 3 minutes. Beat in the eggs, one at a time, until just combined. With the mixer on medium-low speed, beat in the flour mixture in three batches, alternating with the buttermilk, until combined. Stir in the vanilla. Gently fold in the carrots, raisins and roughly chopped walnuts with a rubber spatula.
Divide the batter among the cake pans and bake until a toothpick inserted into the centers comes out clean, about 30 minutes. Transfer to racks and let cool 20 minutes in the pans. Remove the cakes from the pans, remove the parchment and let cool completely on the racks.
Meanwhile, make the frosting: Beat the butter, cream cheese, confectioners' sugar and vanilla in a large bowl with a mixer on medium speed until smooth and fluffy, about 4 minutes.
Place one cake layer on a platter or cake stand and spread 1 cup frosting on top. Add another cake layer, spread with another 1 cup frosting and then add the last cake layer. Cover the top and sides of the cake with a very thin layer of frosting (this is the crumb coat). Refrigerate 1 hour.
Cover the top and sides of the cake with the remaining frosting, then carefully press the finely chopped walnuts into the sides.
Use the large holes on a box grater or a food processor with a shredder attachment to grate the carrots. Pre-shredded carrots are too thick.