PREHEAT oven to 350°F. Coarsely chop 8 of the chocolate squares; set aside. Microwave remaining 8 chocolate squares in large microwaveable bowl on HIGH 2 min., stirring after 1 min. Stir until chocolate is completely melted. Add sugar, butter, eggs and vanilla; stir until well blended. Add flour and baking powder; mix well. Stir in chopped chocolate and walnuts. (Note: If omitting nuts, increase flour to 3/4 cup to prevent excessive spreading of cookies as they 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 1 min.; remove from baking sheets to wire racks. Cool completely.