In large bowl, combine butter, powdered sugar and brown sugar; beat until fluffy. Add maple flavor and egg; beat well. Lightly spoon flour into measuring cup; level off. Add flour and walnuts; beat just until combined. Dough will be soft.
Spoon half of dough onto sheet of plastic wrap. Wrap; shape into disk. Repeat with remaining dough. Place dough in freezer for 20 to 30 minutes or until firm.
Heat oven to 350°F. On floured surface, roll 1 disk of chilled dough to 1/4-inch thickness. With 3-inch maple leaf-shaped cookie cutter, cut out cookies; place on ungreased cookie sheets. If dough becomes sticky, wrap and chill briefly in freezer.
Bake cookies at 350°F. for 7 to 9 minutes or until edges are golden brown. Immediately remove from cookie sheets; place on wire racks. Cool 15 minutes or until completely cooled.
Place melted chocolate in small resealable plastic bag. Cut tiny hole in corner of bag. Squeeze chocolate onto cookies to make veins on leaves.