Step 1: Soak 1 cup raisins in 1/2 cup dark rum, preferably overnight, until they are nice and plump.
Step 2: Preheat the oven to 325 F. Butter two 9-inch cake pans; line the bottoms with parchment. Butter the paper and flour the pans.
Step 3: Spread 1 cup walnuts on a baking sheet and toast for 8 minutes, until fragrant. Cool and finely chop.
Step 4: In a bowl, whisk 2 cups all-purpose flour, 2 teaspoons baking powder, 2 teaspoons baking soda, 1 teaspoon cinnamon and 1 teaspoon salt.
Step 5: In a separate small bowl, whisk 1 cup vegetable oil, 1/2 cup buttermilk and 1 1/2 teaspoons vanilla.
Step 6: Using an electric mixer, beat 4 large eggs and 2 cups sugar at high speed until pale, about 5 minutes. Beat in the liquid ingredients, then beat in the dry ingredients until they're just moistened. Stir in 1 pound shredded carrots, 1 cup toasted walnuts and 1 cup rum-soaked raisins.
Step 7: Divide the batter between the two pans and bake the cakes for 55 minutes to an hour, until springy and golden. Let the cakes cool on a rack for 30 minutes, then unmold the cakes and let cool completely.
Step 1: In a large bowl, using an electric mixer, beat 2 sticks softened butter and 28 ounces softened cream cheese at high speed until light, about 5 minutes.
Step 2: Beat in 1 tablespoon vanilla and the zest of 1 orange, then 2 cups powdered sugar; beat at low speed until incorporated. Increase the speed to high and beat until light and fluffy, about 3 minutes.
Step 3: Peel off the parchment paper and invert one cake layer onto a plate. Spread with a slightly rounded cup of the frosting. Top with the second cake layer, right-side up. Spread the top and sides with the remaining frosting and refrigerate the cake until chilled, about 1 hour. Garnish finished cake with 1/2 cup walnuts, if desired.