In a large microwaveable bowl, melt 160g chocolate in 30-second increments, stirring well after each, until smooth.
Line a 20-cm spring form cake tin with cling wrap.
Pour the chocolate into the base of the tin, smooth with a spoon and place in the freezer.
In a large bowl, beat 300ml of cream just until soft peaks form. Add the Top n Fill and mix to combine. Pour half the caramel mixture over the chocolate base and return to the freezer for 2 hours. Cover remaining caramel with cling wrap and refrigerate.
Once caramel layer is nice and firm, remove from freezer and cut bananas in 2-cm chunks and place over the top.
Melt another 60g of chocolate in the microwave until smooth. Add the chocolate layer. Sit for 2 minutes, then top with remaining caramel. Smooth the top once again. Return to the freezer to completely set, at least 2 hours.
Before serving whip the remaining 250ml of cream and smooth over the top of the ice-cream cake.