Meringues: Whip egg whites with electric mixer until soft peaks form. Gradually add granulated sugar, whipping until stiff peaks form. Stir in vanilla. Fold in powdered sugar in three additions. Fold 1.5 oz. of the OREO Pieces. Spoon mixture into pastry bag fitted with plain 1/2-inch diameter tip.
Pipe mixture onto parchment paper-lined sheet pans to form 52 round cookies. Sprinkle with remaining OREO Pieces. Bake in 200°F standard oven 1 hour 45 min. or until fully dried in center. Remove from oven and cool completely.
Filling: Heat cream in small saucepan on medium-low heat just to boiling. Pour over chocolate; let stand 3 min. Begin stirring chocolate mixture from the center outwards until smooth and shiny in appearance. Cool mixture over a cool water bath, without stirring, until thick.
When ready to serve, spoon Filling into pastry bag fitted with plain 1/2-inch diameter tip. Pipe 1 tsp. onto flat side of 1 cookie. Top with second cookie; gently press together.