Fold OREO Cookie Pieces, base cake and peppermint extract into cake batter. Use #24 scoop to portion batter evenly into 12 paper-lined 2-1/2-inch muffin cups. Bake as directed in recipe for cupcakes. Cool completely in pan.
Meanwhile, cook 2/3 of the sugar and water in small heavy saucepan on medium heat until mixture comes to boil, stirring constantly. Cook until mixture reaches 240°F on candy thermometer, washing down side of pan as necessary. (Do not stir after sugar syrup comes to boil.)
Beat egg whites and remaining sugar in bowl with mixer fitted with whip attachment on medium-high speed until medium stiff peaks form.
Add sugar syrup gradually to egg whites, beating until mixture has cooled to room temperature. Gradually beat in butter until light and fluffy. Stir in base cake and vanilla; spoon into pastry bag fitted with #4 fluted tip.
Remove cupcakes from pan. Pipe butter cream onto tops of cupcakes. Sprinkle each with 1/2 tsp. chopped candy.