Grind the biscuits in the food processor until you have fine crumbs.  Mix the crumbs with demerara sugar in a medium sized bowl.

Gently melt the butter in a small saucepan. Allow to cool slightly then pour into the sugar/biscuit crumbs and combine.

Spoon the crumb mixture into the holes of the tin(s) evenly then using fingers push the crumbs down into the tin to make a solid base around 10mm thick.  Chill in the fridge for at least one hour.

Meanwhile gently heat the condensed milk in a small saucepan stirring all the time until it starts to thicken.  This will take around 20 minutes.

When the biscuit bases have gone solid in the fridge, use the tip of a knife to coax them out of the tin and arrange on a plate.

Let the condensed milk cool slightly then decant into a disposable piping bag with the end snipped off.  Whilst still soft, pipe spirals of condensed milk onto the biscuit bases.

Finally slice rings of banana to push on top and give a light sprinkling of ground cinnamon.