HEAT oven to 350°F.
CHOP 8 chocolate squares (1 pkg.); set aside. Coarsely chop remaining chocolate squares; place in large microwaveable bowl. Microwave on MEDIUM 3 to 4 min. or until chocolate is almost melted; stir until completely melted. Add butter, sugar, eggs, flour and baking powder; stir until well blended. Stir in remaining chopped chocolate and nuts.
DROP 2 tablespoonfuls of dough, 2 inches apart, onto baking sheets.
BAKE 12 to 13 min. or until cookies are soft to the touch but tops are still slightly shiny. Cool on baking sheets 1 min.; transfer to wire racks. Cool completely.