Pulse the biscuits in a food processor until finely crushed. Tip into a bowl and stir through the melted butter. Roughly line 6 x 8-9cm (3-3½in) fluted tart cases with a square of cling film (large enough to hang over the edges).
Divide the biscuit mix between the cases, pressing evenly and firmly with the back of a spoon. Chill for at least 2 hours or ideally overnight.
Use the clingfilm to ease the bases out, then discard the clingfilm. Spread the caramel sauce on top, then add all but 6 of the banana slices, dividing evenly between the pies.
Whisk the cream and vanilla to soft peaks, then spoon a dollop onto each pie.
Finish each with a grating of dark chocolate and a reserved slice of banana. Serve immediately, or chill until ready to eat (the pies will keep for up to 24 hours).