Baklava: Set aside 7 phyllo sheets and cover with dampened towel. Line 1/4 sheet pan with parchment paper.
Using remaining phyllo sheets, place 1 sheet onto prepared pan; brush lightly with butter. Top with 6 additional phyllo sheets, brushing each lightly with butter. Sprinkle top with a thin layer of combined OREO Pieces and OREO Base Cake.
Top with 3 more phyllo sheets, brushing each lightly with butter. Sprinkle top sheet lightly with OREO mixture. Continue alternating layers of 3 additional buttered phyllo sheets and OREO mixture, until all but the reserved 7 phyllo sheets have been used.
Top with 7 reserved phyllo sheets, brushing each with remaining butter, including the top layer. Refrigerate pan 15 min.
Remove pan from refrigerator and cut into 2-1/2-inch squares. Cut squares diagonally in half to form triangles. Bake in 300°F standard oven 1 hour 45 min. Cool pan on wire rack at least 1 hour.
Syrup: When baklava is cool, combine all syrup ingredients in saucepan. Bring just to boiling. Pour over cooled baklava. Let stand at room temperature until all of the syrup has been absorbed.