Stir together first 4 ingredients in a large bowl until blended.
Combine flour and next 3 ingredients; add to butter mixture, stirring until blended. Stir in apple slices and 1 cup walnuts. Spread into a greased 13- x 9-inch pan.
Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Cool completely in pan on a wire rack. Spread with Cream Cheese Frosting; sprinkle with walnuts, if desired.