Cream butter; gradually add sugar, beating well. Add egg and vanilla, beating until smooth. Combine flour, soda, and salt; stir into creamed mixture. Spread evenly in a lightly greased 13- x 9- x 2-inch baking pan. Bake at 350° for 15 minutes. Cool in pan 10 minutes.
Spread filling evenly over crust. Combine remaining ingredients; sprinkle over filling. Bake at 350° an additional 15 minutes. Cool and cut into 2- inch squares.