Preheat oven to 350.
Lightly coat sides and bottom of two 9-inch cake pans with nonstick spray.
Line bottoms with parchment paper rounds; lightly coat paper with more nonstick spray.
Combine carrots and buttermilk in a medium bowl.
In the bowl of a stand mixer, mix the oil and both sugars on speed 2 until the mixture is no longer lumpy, about 1 minute, scraping the bowl down after 30 seconds.
Add the eggs one at a time, mixing for 30 seconds on speed 2 after each addition.
Scrape sides and bottom of bowl after each egg.
Continue mixing until mixture thickens and lightens in color.
Stir in vanilla.
Whisk flour, cinnamon, nutmeg, baking powder, baking soda, and salt in a large bowl.
Add half of the dry ingredients followed by half of the carrot mixture to the stand mixer, stirring until smooth.
Scrape sides and bottom of the bowl.
Repeat with remaining dry ingredients and carrot mixture.
Stir in walnuts with a rubber spatula.
Divide batter between prepared pans.
Bake cakes, rotating pans at the 20 minute mark.
Check for doneness after 35 minutes.
When done, transfer pans to a cooling rack for 10 minutes.
Remove cakes from pans, and cool thoroughly before frosting.
