In large bowl cream butter and granulated sugar until light and fluffy; beat in egg and vanilla. In small bowl combine flour, baking powder and salt; blend into creamed mixture. Reserve half of dough. Stir chocolate into remaining dough. Wrap and refrigerate doughs 30 minutes. Working with one dough at a time on lightly floured surface, roll or pat into 7×4-1/2-inch rectangle. Cut dough into 9 (7×1/2-inch) strips. Repeat with remaining dough. Place one strip chocolate dough on sheet of plastic wrap. Place one strip vanilla dough next to chocolate dough. Place second strip of chocolate dough next to vanilla dough to make bottom layer. Prepare second row by stacking strips on first row, alternating vanilla dough over chocolate, and chocolate dough over vanilla. Repeat with third row to complete 1 bar. Repeat entire process with remaining dough strips, starting with vanilla dough, to complete second bar. Wrap both bars and refrigerate 1 hour. Preheat oven to 350°F. Lightly grease cookie sheets. Cut bars crosswise into 1/4-inch slices. Place 2 inches apart on prepared cookie sheets. Bake 10 to 12 minutes. Cool on cookie sheets 2 minutes; cool completely on wire racks. In small bowl combine powdered sugar and water until smooth. Using icing to attach, decorate cookies with "M&M's"® Chocolate Mini Baking Bits to look like Tic-Tac-Toe games. Store in tightly covered container.
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