Cream butter and brown sugar until light and fluffy. Beat in egg and molasses. In another bowl, whisk flour, pie spice, baking soda, ginger and salt; gradually beat into creamed mixture. Refrigerate, covered, until easy to handle, about 2 hours.
Preheat oven to 325°. On a lightly floured surface, roll dough to 1/8-in. thickness. Cut with a floured 3-in. tree-shaped cookie cutter. Place 2 in. apart on ungreased baking sheets. Gently press M&M's into half of the cookies. Bake until edges are firm, 8-10 minutes. Remove to wire racks to cool completely.
If using vanilla frosting, tint with green food colouring if desired. Spread frosting over bottoms of plain cookies; covered with decorated cookies. Store in an airtight container.