PREHEAT oven to 325°F. Line baking sheets with parchment paper or lightly grease.
MELT 2/3 cup morsels in microwave-safe bowl on MEDIUM-HIGH (70%) power for 30 seconds; STIR. Morsels may retain some of their original shape. Microwave at additional 10- to 15-second intervals, stirring just until morsels are melted. Set aside.
SIFT flour, cocoa, baking soda and salt into medium bowl. Beat butter, brown sugar and granulated sugar in large mixer bowl until creamy. Add melted chocolate and mix well. Add egg and vanilla extract, mixing until well blended, about 1 minute. Add flour mixture, mixing just until blended. Stir in remaining 1 cup morsels. Drop dough by level 1/4-cup measure 3 inches apart onto prepared baking sheets.
BAKE for 16 to 18 minutes or until wooden pick inserted in center comes out with moist crumbs and the tops have a cracked appearance. Cool on baking sheets for 5 minutes. Remove to wire rack to cool completely.