In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine flour and baking powder; gradually add to creamed mixture and mix well.
Divide dough in half. Tint half of the dough green; knead well to distribute color evenly. Leave remaining dough white. Cover and refrigerate for 1 hour or until easy to handle.
On a lightly floured surface, roll out each portion of dough to 1/8-in. thickness. Use tree-shaped cookie cutters with identical shapes, measuring 2-1/2 in., 3 in., 4 in. and 4-1/2 in. Cut out an even number of cookies with each size cookie cutter.
Place 1 in. apart on ungreased baking sheets. Bake at 350° for 8-10 minutes or until golden brown. Immediately cut half of each size tree cookies in half from top to bottom. If tree cookie cutters have trunks, trim trunks off trees, creating a flat base. Remove to wire racks to cool.
For frosting, in a large bowl, combine the confectioners' sugar, water, meringue powder, vanilla and cream of tartar. Beat on high speed for 8-10 minutes or until stiff peaks form. Divide frosting in half. Tint half of the frosting green; leave remaining frosting white. Cover frosting with damp paper towels or plastic wrap between uses.
To make four-sided garland trees (on top in photo at left): Cut a small hole in the corner of a pastry or resealable plastic bag; insert round pastry tip #5. Fill with green or white frosting to match the tree you are decorating. Pipe a line of frosting along the cut edge of one of the halved cookies (see Photo 1); press frosted edge along center of a matching whole cookie. Repeat. Let dry until firm.
Stand up partially assembled trees. Attach one matching cookie half to the opposite side of the tree by piping frosting along the center of the whole cookie (see Photo 2). Let dry completely.
Using another pastry or resealable plastic bag, medium star pastry tip #21 and either white or green frosting, pipe garlands around sides of trees. Decorate with sprinkles if desired. Let dry completely.
To make fuller trees (top right and lower left in photo at left): Assemble trees as described for four-sided garland trees, using white or green cookies. Using matching colored frosting, attach four cookie halves, one size smaller than assembled tree cookies. Let trees dry completely. Decorate using either the garland or snow-tipped technique. Dry completely.
To make snow-tipped trees (at lower left in photo at left): Assemble trees as described for fuller trees using green cookies. Using another pastry or resealable plastic bag, large round pastry tip #8 and white frosting, pipe snow on tips of branches. Sprinkle with edible glitter. Let dry completely.
To make glazed trees (at lower right in photo at left): In a small bowl, thin 1 cup green frosting with 2-3 tablespoons water until frosting reaches pourable consistency. Place matching pairs of whole and halved cookies on a wire rack over waxed paper. Pour icing over cookies; spread with a metal spatula to completely cover top and sides of cookies. Let dry completely. Assemble, using garland tree method; let dry. Decorate as desired. Yield: 6 dozen cookies (or 18-36 cookie trees, depending on fullness of trees).