Line base of a 20cm round springform tin with parchment paper, then grease base and sides.
For the base: Combine butter with biscuits and mix well. Press firmly into prepared tin. Chill for at least 1 hour.
Meanwhile, make the filling. Gently heat butter and sugar until sugar has dissolved. Pour in condensed milk and gently bring to the boil, stirring continuously until thick. Do this slowly, to avoid getting any lumps. Immediately remove from heat and allow to cool until just warm. Spread on biscuit base and chill for 1 hour.
Just before serving, fold 3/4 of the sliced bananas into the whipped cream. Spoon over toffee layer. Decorate with remaining bananas. Dust with cocoa powder and serve.