Preheat oven to 375°. In a large bowl, cream butter and confectioners' sugar until blended. Beat in egg and vanilla. In another bowl, whisk flour, gelatin, baking soda and cream of tartar; gradually beat into creamed mixture. , Divide dough into 4 portions. Shape each into a disk; cover. Refrigerate 30 minutes or until firm enough to roll., Preheat oven to 375°. For filling, in a small bowl, beat jelly and confectioners' sugar. On a lightly floured surface, roll 1 portion of dough to 1/8-in. thickness. Cut with a floured 2-in. round cookie cutter. Place about 1/2 teaspoon filling in center of half of the cookies. Cover with remaining cookies. Press edges with a fork to seal. Place 2 in. apart on ungreased baking sheets. Repeat with remaining dough and filling., Bake 10-12 minutes or until edges are light brown. Cool on pans 2 minutes. Remove to wire racks to cool completely., In a small bowl, mix confectioners' sugar and milk until smooth. Drizzle over cookies; let stand until set. Store in an airtight container in the refrigerator.