Preheat oven to 350 degrees F. Grease a 13x9x2-inch baking pan; set aside.
In a large bowl whisk eggs, oil, brown sugar, orange zest, orange juice, and vanilla until combined. Whisk in flours, baking powder, cinnamon, soda, salt, and nutmeg until combined. Stir in carrots and walnuts. Pour into prepared pan.
Bake about 30 minutes or until a wooden toothpick inserted in center comes out clean. Cool in pan on a wire rack. Spread with Cream Cheese Frosting.
Cream Cheese Icing
In a large mixing bowl beat cream cheese and butter with an electric mixer on medium speed until combined and smooth. Add powdered sugar; beat on low until combined. Stir in orange zest.