Process biscuits until fine. Transfer biscuits to a medium bowl, stir in butter. Press mixture evenly over base and side of 20cm springform pan. Refrigerate while preparing filling.
In a small saucepan, combine brown sugar, extra butter and 2 tablespoons of the cream. Stir over medium heat until sugar dissolves. Simmer 5 minutes.
In a small heatproof jug, sprinkle gelatine over the water. Stand jug in a small saucepan of simmering water. Stir until dissolved.
In a medium bowl, beat cream cheese and caster sugar with an electric mixer until smooth.
In a small bowl, whip the remaining cream with an electric mixer until soft peaks form. Stir slightly warm gelatine mixture into cream cheese mixture with the Mars Bars; then fold in the whipped cream.
Pour 2/3 of the cream cheese mixture onto the biscuit base in pan. Drizzle over 2/3 of the butterscotch sauce. Swirl the mixture to distribute the butterscotch sauce. Repeat the process with remaining cream cheese mixture, butterscotch sauce.
Refrigerate for 3 hours or until set. While cake is setting, combine chocolate and extra cream in a small saucepan; stir over low heat until smooth. Serve the cheesecake drizzled with milk chocolate sauce.