Place 1 packet of Tim Tams in a food processor and process until resembles fine crumbs. Roughly chop remaining packet.
Place butter and half the can of condensed milk in a small saucepan. Heat gently stirring until combined. Add butter mixture to the bowl of crushed Tim Tams, stir well, then add the chopped Tim Tams, mix and press into tin. Chill for at least 30 mins.
In a medium size microwave proof bowl or jug, add chocolate and vegetable oil, microwave on 600w power for approx. 2 mins stirring halfway through. Add remaining condensed milk, stir to combine and microwave on 600w power for 1 minute. Pour mixture over biscuit base.