HEAT oven to 200ºF.
COVER 2 baking sheets with parchment. Draw 24 (1-1/2-inch) circles on each parchment sheet.
GRATE 1 oz. chocolate. Beat egg whites and cream of tartar in medium bowl with mixer on high speed until soft peaks form. Add sugar, 1 Tbsp. at a time, beating constantly until stiff peaks form. Gently stir in vanilla and grated chocolate.
SPOON egg white mixture into resealable plastic bag; cut 1/2-inch piece off one bottom corner. Use circular motion to squeeze about 1 Tbsp. egg white mixture over each circle on parchment.
BAKE 1 hour 10 min. or until meringues are dry and can easily be transferred from parchment to wire racks; cool completely. Meanwhile, melt remaining chocolate as directed on package; stir in peanut butter. Cool.
SPREAD flat sides of 24 cookies with peanut butter mixture, adding about 1 tsp. to each. Cover with remaining cookies, flat sides down.