In large bowl, beat butter and brown sugar until light and fluffy. Add egg and vanilla, beat until blended. Add flour, cracker crumbs and salt. Beat on low speed 1 minute or until stiff dough forms.
Shape dough by heaping teaspoonfuls into 60 balls. On ungreased cookie sheets, place balls 2 inches apart. With thumb, make indention in center of each.
Bake 10-12 min or until cookies are firm and edges are just beginning to brown. Lightly press 2 marshmallows in center of each cookie; bake 2-3 minutes longer. Top marshmallows on each cookies with 1 rectangle of chocolate' let stand till soft, smear with knife. Let stand until chocolate is set.