To prepare cake, combine first 7 ingredients in a bowl; beat with a mixer at low speed for 1 minute. Scrape sides of bowl; beat at medium-high speed for 2 minutes or until batter is smooth, scraping sides of bowl occasionally. (Batter will be thick.) Fold in carrot, raisins, and walnuts. Spoon batter evenly into 2 (9-inch) round cake pans coated with cooking spray.
Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes; remove from pans. Cool completely on a wire rack.
To prepare frosting, place cream cheese, milk, and vanilla in a large bowl; beat with a mixer at high speed until creamy. Gradually add powdered sugar; beat at low speed just until blended.
Place 1 cake layer on a plate; spread with 1/2 cup frosting. Top with remaining cake layer. Spread remaining frosting over top and sides of cake.