Chop 8 oz. chocolate; 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. Stir in sugar, eggs and vanilla.
Add flour and baking powder; mix well. Stir in chopped chocolate and nuts. (Note: If omitting nuts, increase flour to 3/4 cup to prevent excessive spreading of cookie dough as cookies bake.)
Drop rounded tablespoonfuls of dough, 2 inches apart, onto baking sheets.
Bake 12 to 13 min. or until cookies are puffed and shiny. Cool on baking sheets 1 min.