Pudding Mix: Whisk cold milk, water, and condensed milk until smooth. Whisk in the instant pudding mix until thickened.
Chill Base: Place the pudding mixture in the freezer for 1 hour. It should be very cold and thick, but not frozen.
Whip Cream: In a cold bowl, whip heavy cream and vanilla extract until soft peaks form.
Fold: Gently fold the chilled pudding mixture into the whipped cream in batches using a spatula until smooth.
Layer: In a large dish, layer pudding cream, then bananas, then Biscoff biscuits. Repeat until the dish is full, ending with cream on top.
Set: Cover and refrigerate for at least 4 hours (overnight is best).
Serve: Decorate with fresh banana slices, crushed cookies, and caramel drizzle right before serving.