Preheat oven to 350 F. While chocolate cools, beat butter in a medium bowl on medium-high speed 2 minutes or until smooth. Add granulated sugar, beating until creamy. Add egg, egg yolk, and vanilla, beating well. Stir in melted chocolate. Stir in flour and chopped walnuts. Cover and chill 1 hour.
Shape dough into 1-inch balls. Place on cookie sheets lined with parchment paper. Press a walnut half into top of each cookie. Bake for 10 to 12 minutes or until edges are lightly browned. Remove from pan and cool completely on a wire rack.
Spoon Chocolate Glaze evenly over cooled cookies. Let stand until glaze is set. Dust with powdered sugar, if desired. Store the cookies in an airtight container between layers of waxed paper for up to 1 week or freeze for up to 1 month.
Melt sweet baking, bittersweet, or semisweet chocolate and unsalted butter in a small saucepan. Remove from heat. Add vanilla, powdered sugar, and nonfat milk, and blend well.