Position a rack in the center of the oven and heat the oven to 350 degrees F. Lightly oil and flour the sides of two 9x2-inch round cake pans, tapping out any excess flour. Line the bottoms of the pans with parchment.

In a medium bowl, whisk the flour, cinnamon, baking soda, nutmeg, ginger, and salt. In a large bowl with a hand mixer or in a stand mixer fitted with the paddle attachment, mix the oil, eggs, carrots, brown sugar, walnuts, raisins, and vanilla on medium speed until well blended, about 1 minute. Add the dry ingredients and mix on low speed until just blended, about 30 seconds. Divide the batter evenly between the prepared pans.

Bake until the tops of the cakes spring back when lightly pressed and a cake tester inserted into the centers comes out clean, 28 to 30 minutes.

Let cool in the pans on a rack for 15 minutes. Run a knife around the inside edge of the pans to loosen the cakes, invert them onto the rack, remove the pans, and carefully peel away the parchment. Set the cakes aside to cool completely before frosting.

In a large bowl, beat the cream cheese and butter with the mixer on medium speed until very smooth and creamy, about 1 minute. Add the confectioners' sugar, vanilla, and salt and beat on medium high until blended and fluffy, about 2 minutes. Cover the frosting and set aside at room temperature until the layers are completely cool.

Carefully set one cake upside down on a large, flat serving plate. Using a metal spatula, evenly spread about 1-1/2 cups of the frosting over the top of the cake. Top with the remaining cake layer, upside down. Spread a thin layer (about 1/3 cup) of frosting over the entire cake to seal in any crumbs and fill in any gaps between layers. Refrigerate until the frosting is cold and firm, about 20 minutes. Spread the entire cake with the remaining frosting.

Refrigerate the cake for at least 4 hours or up to 2 days. The cake is best served slightly chilled or at room temperature.

Make Ahead Tip: The flavors of this moist cake only improve with time, so feel free to bake and frost the cake up to a few days ahead.