In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. In another bowl, mix ground pretzels, flour and baking powder; gradually beat into creamed mixture. Divide dough in half. Shape each into a disk; wrap in plastic. Refrigerate until firm enough to roll, about 1 hour.
Preheat oven to 350°. On a lightly floured surface, roll each portion of dough to 1/4-in. thickness. Cut with a floured 3-1/2-in. tree-shaped cookie cutter. Using a floured 1-3/4-in. tree-shaped cookie cutter, cut out the centers of half of the cookies. Place solid and window cookies 1 in. apart on ungreased baking sheets.
Bake 8-10 minutes or until edges are light brown. Remove from pans to wire racks to cool completely.
Spread melted chocolate onto bottoms of solid cookies; let stand until firm. Spread jam over cooled chocolate; top with window cookies. Dust lightly with confectioners' sugar. Freeze option: Freeze undecorated cookies in freezer containers. To use, thaw in covered containers and decorate as directed.