For OREO Ganache: Heat water to boiling. Combine chocolate, OREO Base Cake and cookie pieces, cocoa, salt and vanilla in large bowl. Pour water over mixture and let stand 3 min.
Transfer mixture to food processor or blender. Blend 20 seconds. Pour into clean bowl. Refrigerate several hours or overnight.
For Cookies: Cream butter, sugar and vanilla in bowl of electric mixer fitted with paddle attachment 3 min. or until smooth. Gradually add eggs, beating well after each addition. Stir in combined flour, baking powder and salt until well blended.
Use #30 scoop to portion dough onto parchment paper-lined full sheet pan. For each stuffed cookie, flatten 1 scoop of dough to a 3-inch circle. Place 0.3 oz. OREO Small Cookie Pieces in center of the circle, leaving a 1/4-inch edge. Flatten a second scoop of dough and place over the cookie pieces. Gently seal edges. Arrange 6 stuffed cookies on parchment paper-lined half sheet pans.
Bake in 325°F convection oven 15 to 18 min. or until light golden brown. Cool completely on pan.
For Garnish: Spread tops of each cookie with 0.45 oz. OREO Ganache to within 1/4-inch of the edge, then sprinkle with 0.5 oz. cookie pieces.