Heat oven to 375°F. In large bowl, beat brown sugar and butter with electric mixer on medium speed until light and fluffy. Beat in vanilla and egg until well blended. On low speed, beat in remaining ingredients except walnuts and baking chips until well blended.
On ungreased cookie sheets, drop dough by rounded teaspoonfuls 2 inches apart. Press 1 walnut half onto each cookie.
Bake 6 to 9 minutes or until edges are set and light golden brown. Immediately remove from cookie sheets to cooling racks. Cool completely, about 5 minutes.
Meanwhile, place baking chips in small resealable freezer plastic bag; seal bag. Microwave on High 30 to 45 seconds or until softened. Squeeze bag until mixture is smooth. (If necessary, microwave 30 seconds longer or just until all chips are melted.) Cut small tip from one bottom corner of bag. Squeeze bag to drizzle melted chips over cookies.