In a large bowl, cream butter and brown sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine the flour, baking powder, baking soda and cream of tartar; gradually add to the creamed mixture and mix well (bowl will be full.)
Drop by level tablespoonfuls 2 in. apart onto ungreased baking sheets. Bake at 350° for 10-12 minutes or until golden brown. Remove to wire racks to cool.