In a large bowl, cream butter and brown sugar until light and fluffy. Beat in eggs, water, oil and vanilla. In another bowl, whisk flour, cream of tartar, baking soda and salt; gradually beat into creamed mixture., Divide dough into four portions. Shape each into a disk; wrap in plastic. Refrigerate 2 hours or until firm enough to roll., Preheat oven to 350°. On a lightly floured surface, roll each portion of dough to 1/8-in. thickness. Cut with a floured 2-1/4-in. fluted square cookie cutter. Place 1 in. apart on greased baking sheets., Bake 7-9 minutes or until bottoms are light brown. Remove from pans to wire racks to cool completely., For icing, in a small bowl, beat butter, meringue powder, cream of tartar and salt until blended. Beat in confectioners' sugar alternately with enough water to reach a spreading consistency. Spread over cookies. Let stand until set.