In large bowl cream the butter and sugars until light and fluffy with an electric mixer. Beat in the egg and vanilla.

In small bowl, combine the flour, baking powder and salt and then blend into creamed mixture. Wrap the dough in plastic wrap and refrigerate for 2 to 3 hours.

Preheat the oven 375°F. Working with half of dough at a time, roll the dough on a lightly floured surface to a thickness of about 1/8 inch. Cut into pairs of desired shapes. Use 3-inch cookie cutters for Christmas trees, and 2 1/2-inch cutters for stars, or thereabouts. Re-roll the trimmings and cut out more cookies. Put the cutouts on ungreased cookie sheets with about 1 inch between them. Bake for 7 to 9 minutes or until lightly browned. While the cookies are still hot and right out of the oven, immediately cut 1 cookie of each pair in half vertically. Transfer the whole and halved cookies to wire racks to cool completely.

Prepare Decorating Glaze: Whisk together 3 cups of powdered sugar and 3 large egg whites in a bowl until smooth. Tint the glaze with food coloring as desired.

Pour the glaze over the cookies and let it set. Spread line of glaze along the cut edge of the cookies to act as glue. Press the halved cookies into both sides of the whole cookie that matches it to make a three-dimensional cookie. Let set. Repeat with the remaining cookies. 

Using glaze to attach them, decorate cookies with "M&M's"® Chocolate Mini Baking Bits. Store in a tightly covered container.