Step 1 Grease and line 18 x 28 cm slice tin with baking paper, extending 2 cm above tin edges. Process one pkt of Tim Tam in a food processor until finely crushed place in a medium bowl and set aside.
Step 2 Meanwhile roughly chop remaining packet ready to add to mixture.
Step 3 Place butter and half the can of condensed milk in a small saucepan stir over medium heat until melted and combined.
Step 4 Add butter mixture to the bowl of crushed Tim Tam biscuits, stir well and add in the chopped Tim Tam, mix and press into tin. Chill for at least 30 mins.
Step 5 In a medium size microwave proof bowl add chocolate and oil, microwave on 600w power for approx. 2 mins stirring halfway. Add remaining condensed milk.
Step 6 Stir to combine and microwave on 600 w power for 1 minute. Pour mixture over biscuit base. Chill and cut into 24 squares.