Grease a 20 cm round springform pan; line base and side with baking paper. Process OREO cookies to fine crumbs. Add butter, process until combined. Press mixture onto base of pan using the back of a spoon. Refrigerate for 20 minutes.
Meanwhile, for cheesecake layers, place ¼ cup water in a small heatproof bowl, sprinkle over the gelatine. Stand bowl in boiling water, stirring until gelatine is dissolves.
Beat PHILADELPHIA Cream Cheese, milk and sugar in a medium bowl with an electric mixer until smooth. Divide mixture into three even portions.
Fold melted CADBURY Baking White Chocolate into one portion, melted CADBURY Baking Milk Chocolate into another and melted CADBURY Baking 70% Cocoa Dark Chocolate into remaining portion. Fold one-third of the gelatine mixture into each portion. Gently fold one-third of the whipped cream into each portion.
Spread dark chocolate mixture evenly over OREO cookie base, smoothing the surface. Spread milk chocolate mixture over dark chocolate layer, followed by the white chocolate mixture, being careful not to combine the three layers. Refrigerate for 3 hours or until set
To make chocolate topping, combine extra cream and CADBURY Baking 70% Cocoa Dark Chocolate in a microwave-safe bowl, microwave on 80% power for 60-90 seconds, in bursts, until almost melted. Cool slightly, then stir until slightly thickened
To decorate, spoon chocolate topping over the top of the cheesecake, pushing it slightly over the edge. Scatter with CADBURY FLAKE bar pieces and broken TOBLERONE bars.