Preheat oven to 350°. Line bottoms of 2 greased 9-in. round baking pans with parchment; grease parchment., Place apples in a food processor; pulse until finely chopped. In a large bowl, beat sugar, oil, eggs and vanilla until well blended. In another bowl, whisk flour, pie spice, baking powder and salt; gradually beat into sugar mixture alternately with buttermilk. Stir in apples and walnuts., Transfer to prepared pans. Bake until a toothpick inserted in center comes out clean, 35-40 minutes. Cool in pans 10 minutes before removing to wire racks; remove paper. Cool completely., In a large bowl, beat cream cheese, butter, sugars, vanilla and pie spice until smooth. Spread frosting between layers and over top and sides of cake. Gently press walnuts into frosting on top of cake. Refrigerate leftovers.