In large bowl, beat butter and sugar with electric mixer on medium speed about 30 seconds or until smooth. Add 1/2 cup walnuts and the egg yolk; beat until blended. Add flour, baking powder, salt and maple flavor. On low speed, beat until stiff cookie dough forms. Shape dough into a ball. Wrap in plastic wrap; refrigerate 45 minutes.
Heat oven to 350°F. Divide dough into 8 equal parts. On lightly floured surface, shape each part into a rope 12 inches long and 3/4 inch thick. Cut into 2-inch lengths; place about 2 inches apart on ungreased cookie sheets. Flatten cookies slightly.
Bake 15 to 17 minutes or until edges begin to brown. Cool 2 minutes; remove from cookie sheet to cooling rack. Cool completely, about 30 minutes.
In small microwavable bowl, microwave chocolate chips uncovered on High 1 minute 30 seconds, stirring every 30 seconds, until chips can be stirred smooth. In another small bowl, place 1 cup walnuts.
For each cookie, dip 1/2 inch of 1 long side into chocolate, then coat chocolate edge with walnuts. Place on sheets of waxed paper; let stand about 2 hours until chocolate is set.