In a large bowl, combine the flour, brown sugar and pecans. Stir in butter until crumbly. Set aside 1/3 cup for topping. Press remaining mixture into a greased 8-in. square baking pan. Bake at 350° until lightly browned, 12-15 minutes.
Meanwhile, in a large bowl, beat cream cheese and sugar until smooth. Beat in the egg, milk, lemon juice and vanilla. Pour over crust; sprinkle with reserved pecan mixture.
Bake until firm, 20-25 minutes. Cool on a wire rack. Cut into 16 squares, then cut each square in half diagonally. Store in the refrigerator.