In a large mixing bowl, cream the butter and brown sugar. Beat in eggs, molasses and vanilla. Combine the flour, ginger, cinnamon, baking soda, salt and allspice; add to creamed mixture alternately with buttermilk.
Drop by tablespoonfuls 2 in. apart onto greased baking sheets. Bake at 375° for 7-9 minutes or until set. Remove to wire racks to cool.
For frosting, in a small mixing bowl, cream shortening and butter. Beat in vanilla. Gradually beat in confectioners' sugar and ginger. Add milk; beat until light and fluffy. Spread over cookies. Decorate with sprinkles if desired. Store in an airtight container. Yield: 7-1/2 dozen.