Stir together first 4 ingredients in 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 (if using). The batter will be very thick, almost like cookie dough.
Spread into a well-greased 13- x 9-inch pan. (It's pretty sticky, so make sure to grease your pan well!)
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.