Heat oven to 350°F. Line bottom and sides of 13x9-inch pan with foil, extending foil 2 inches on 2 opposite sides of pan.
In large bowl, beat butter and brown sugar with electric mixer on medium speed, scraping bowl occasionally, until fluffy. On low speed, beat in flour, ground ginger and nutmeg until crumbly. Stir in crystallized ginger. Press dough firmly in pan. Prick surface generously with fork.
Bake 18 to 22 minutes or until light golden brown and set. Cool 10 minutes. Remove from pan by lifting foil. To make 24 squares, cut into 6 rows by 4 rows by pressing down with a long knife (do not use sawing motion). Cut each square diagonally in half to make triangles. Cool completely, about 1 hour.
In small microwavable bowl, microwave chocolate uncovered on High 1 minute. Stir; microwave 30 seconds longer, stirring every 15 seconds, until completely melted. Dip one short, flat edge of each triangular cookie in chocolate; if necessary, shake off any excess chocolate. Place on waxed paper; let stand until chocolate is set before storing between layers of waxed paper in loosely covered container.