Preheat oven to 350°F. Lightly coat two 9" pans, three 8" pans, one sheet cake pan, or cupcake pans with cooking spray or my preferred choice, butter or Crisco with an even dusting of flour on top. Cakes will never stick to the pan using the latter method.
If using raisins and rum, heat together in a small saucepan over low heat just until warm, about 2 minutes. Remove from heat and let sit until iquid is absorbed and raisins are plump, about 15 minutes.
Meanwhile, toast walnuts on a rimmed baking sheet, tossing once, until golden brown, about 8-10 minutes; let cool. Combine carrots and buttermilk in a medium bowl.
Whisk flour, cinnamon, ginger, nutmeg, baking powder, salt, and baking soda in a large bowl. Using an electric mixer on high speed, beat eggs, granulated sugar, brown sugar, and vanilla extract until pale and thick, about 4 minutes. Reduce speed to medium-low and gradually stream in oil. Add dry ingredients in 3 additions, alternating with carrot mixture in 2 additions, beginning and ending with dry ingredients; mix until smooth. Fold in raisins and walnuts, if using with a rubber spatula. Scrape batter into prepared pans. For even, flat cake layers, I prefer to use Wilton cake strips while baking.
Bake cakes until a toothpick tester inserted in the center comes out clean, about 35-40 minutes. Transfer pans onto a wire rack and let cool, about 10 minutes. Run a knife around sides of cakes and invert onto a sheet of wax paper. Once cooled, wrap in wax paper and foil and store on an flat, solid surface in the freezer. Let cakes freeze completely.