Four Layer Pumpkin Cake w/ Orange Cream Cheese Frosting

Cake: position rack in bottom third of oven; preheat to 350°. Spray 2 9" cake pan w/ Pam. Line bottoms w/ parchment; spray parchment. Whisk flour and next four ingredients in large bowl. Using electric mixer, beat butter in another large bowl until smooth. Add eggs, one at a time, beating to blend between additions. Beat in pumpkin. Add dry ingredients to butter mixture in 3 additions alternately w/ milk in two additions, beating to blend between additions. Divide batter between the 2 pans. Bake cakes until tester inserted in middle caomes out clean; about 40 minutes. Cool in pans on rack, 15 minutes. Run knife around cakes to loosen. Invert cakes onto racks. Remove parchment, turn cakes over onto racks, top sides up, and cool completely. Frosting: Using electric mixer, beat butter in a large bowl until smooth. Add cream cheese and orange peel; beat until smooth. Add powdered sugar and beat on low speed until smooth. Trim rounded tops from cakes. Using long serrated knife, cut each cake horizontally in half. Place 1 cake layer, cut side up on platter. Spoon ⅔ C frosting atop cake in dollops; spread to edges. Repeat 2 more times; top w/ remaining cake layer, cut side down. Spread remaining frosting over top and sides (layer will be thin). Decorate cake w/ walnuts and serve.