Cream butter and brown sugar until light; add flour and chopped walnuts. Cream with spoon until mixture forms crumbs. Set aside 1 c. mixture for topping. Press remaining crumb mixture into ungreased 8" square pan.
Bake in moderate oven (350°) 12 to 15 minutes. Set pan on rack to cool.
Combine white sugar and cream cheese; beat until smooth. Add egg, mil