Reserve 2 oz. chocolate. Coarsely chop remaining chocolate. Microwave chopped chocolate in small microwaveable bowl on HIGH 2 min. Stir until chocolate is completely melted; cool slightly. Meanwhile, combine flour, baking powder and salt.
Beat butter and sugars in large bowl with mixer until blended. Add eggs, 1 at a time, beating well after each addition. Blend in melted chocolate and vanilla. Gradually add flour mixture, beating well after each addition. Stir in nuts.
Divide dough in half; roll each half into 10x2-inch log on lightly floured surface. Place, 2 inches apart, on baking sheet sprayed with cooking spray.
Bake 40 to 50 min. or until toothpick inserted in centers comes out clean. Cool on baking sheet 15 min. Transfer logs to cutting board. Use serrated knife to cut each log into 18 diagonal slices; place, cut sides down, on baking sheet.
Bake 10 min. on each side or until cookies are lightly browned on both sides. Remove to wire racks; cool completely.
Melt reserved chocolate as directed on package; drizzle over biscotti. Let stand until chocolate is firm.