Whoopie Pies: Draw 42 (2-1/8-inch) circles, spaced 1-1/2 inches apart, on parchment paper-lined sheet pans. Turn paper over so marked side is down.
Sift flour, cocoa powder, OREO Base Cake, baking soda and salt in large bowl; set aside.
Cream butter and sugar with electric mixer fitted with paddle attachment on medium-high speed until fluffy. Mix buttermilk, whole egg and vanilla in separate bowl. Add dries alternately with liquids, beating well after each addition, beginning and ending with dry ingredients.
Spoon batter into pastry bag fitted with #8 or #9 round tip. Pipe circles (no more than 3/8-inch high) onto parchment paper, using prepared templates. Bake in 350°F convection oven (or 375°F standard oven) 13 to 15 min. or until cakes spring back when lightly touched. (Do not overbake.) Transfer immediately to wire racks to cool completely.
OREO Buttercream: Make an Italian buttercream by heating sugar and water in small saucepan to 135°F. Beat egg whites with electric mixer fitted with whip attachment on high speed until soft peaks form. Continue cooking sugar mixture to 140°F. Slowly pour sugar mixture down the side of the bowl while whipping on high speed until stiff peaks form. Continue whipping until bottom of bowl is cool.
Add butter in 4 additions, whipping on high speed until thoroughly blended. Continue whipping on medium speed until light and fluffy. Stir in OREO Pieces.
Assembly: Spoon buttercream into pastry bag fitted with #8 or #9 tip. Pipe 3/4 oz. buttercream onto flat side of 1 cake round. Top with second cake to create sandwich. Repeat with remaining cakes and buttercream to create 21 pies.