Chop 8 oz. chocolate coarsely; set aside. Microwave remaining chocolate in large microwaveable bowl on HIGH 2 min., stirring after 1 min. Stir until chocolate is completely melted.
Add butter; stir until melted.
Add peanut butter, sugar, eggs and vanilla; stir until blended.
Add flour and baking powder; mix well. Stir in chopped chocolate and nuts.
Drop rounded tablespoonfuls of dough, 3 inches apart, onto baking sheets sprayed with cooking spray.
Bake 12 to 14 min. or until cookies are puffed and feel set to the touch. Cool on baking sheets 1 min.