In a bowl, cream butter and sugar. Beat in eggs and vanilla. Combine flour, baking soda and salt; stir into the creamed mixture alternately with buttermilk. Fold in apple and nuts. Pour into a greased 9x5-in. loaf pan. For topping, combine the first four ingredients; cut in butter until crumbly. Sprinkle over batter. Bake at 350° for 55-60 minutes or until bread tests done. Cool in pan 10 minutes before removing to a wire rack.