My sister, Morgan, prefers carrot cake bars to a traditional layer cake. She wants each and every bite to have an equal ratio of cake to frosting, and after trying it, I agree. However, we differ on our opinion of nuts. She is a no nuts in carrot cake girl, whereas I love the texture and toasted flavor and think they balance out the sweetness. I decided to take the nuts out of the batter and add them to the top of the frosting, so that we can both be satisfied. Sprinkled nuts also camouflage a multitude of frosting imperfections and add simple decoration to the top. I double the walnuts on one half and leave them off of the other, making this one recipe perfect for both of us.
Make the cake: Preheat the oven to 350 degrees F (175 degree C). Spray a 9-by-13-inch (23-by-33-centimeter) baking dish with cooking spray.
In a large bowl, whisk together the flour, cinnamon, baking powder, and salt. Set aside. In the bowl of a stand mixer fitted with a paddle attachment, cream the oil and sugar until light and fluffy, about 4 minutes. Add the eggs one at a time and continue to beat until thick and creamy. Add the carrots and mix to combine. Remove the bowl from the mixer and stir in the flour mixture by hand, making sure not to overmix. Pour the batter into the prepared baking dish and bake until the edges pull away from the sides of the pan and a toothpick inserted in the center comes out clean, about 35 to 40 minutes. Let cool completely in the baking dish.
Meanwhile, make the frosting: (If you are extra fancy and have two stand mixers, then you are all set to get started, but if you are like me and just have the one, then you have to wash the bowl and paddle attachment now.) In the bowl of a stand mixer fitted with the whisk attachment, whip the cream cheese and butter until well combined, about 2 minutes. Add the confectioners' sugar 1/2 cup (65 g) at a time and whip on medium-low until homogenous, thick, light, and fluffy. Stir in the vanilla. Using an offset spatula, spread the frosting evenly onto the top of the cooled cake. Sprinkle the top with the walnuts. Cut the cake into thirds lengthwise and then quarters vertically to make 12 bars. Remove the cake with an offset spatula to a platter.
You can also make this into a layer cake by baking the cakes in two 8-inch cake pans instead of one 9-by-13-inch baking dish.