Prepare the pans: Heat the oven to 350°F (175°C). Grease two 9-inch round pans, line with parchment, and grease again.
Combine dry ingredients: Whisk flour, baking powder, baking soda, cinnamon, nutmeg, and salt in a bowl.
Mix wet ingredients: Whisk brown sugar, granulated sugar, eggs, oil, and vanilla until smooth.
Make the batter: Stir the dry ingredients into the wet mixture in two additions, mixing only until combined.
Add the fillings: Fold in grated carrots, golden raisins, and chopped nuts.
Bake: Divide the batter between the pans and bake for 30 to 36 minutes, until a toothpick comes out with a few moist crumbs.
Cool: Rest the cakes in the pans for 15 minutes, then transfer to a rack and cool completely.
Make the frosting: Beat sour cream, cream cheese, and butter until creamy. Gradually add powdered sugar and mix until smooth.
Assemble: Frost the first layer, add the second layer, then cover the top and sides. Garnish with chopped nuts or cinnamon.