Cream butter and margarine in a large mixing bowl; gradually add sugar, beating until light and fluffy. Add egg yolk; beat well. Stir in vanilla. Add flour and salt to creamed mixture; stir well.
Press dough into a greased 15- x 10- x 1-inch jellyroll pan; brush with egg white. Sprinkle pecans on top. Bake at 350° for 30 minutes or until lightly browned. Cut into 2- x 1-inch bars while warm.