Melt the butter in a large saucepan, then stir in the crushed biscuits until they are completely combined and the mixture feels like wet sand. Press firmly into the base and sides of a round 20cm (8in) loose-bottomed tart tin. Chill for 1 hour or until very firm.
Just before serving, remove the pie case from the tin. Spoon the caramel into the case, reserving a little for decoration, and smooth the top. Put the cream, milk and sugar into a large bowl and beat with an electric whisk until thickened but not stiff.
Slice the bananas over the caramel, then top with the cream and a drizzle of the reserved caramel. Using a vegetable peeler, shave the chocolate into shards and curls and scatter over.
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