Preheat oven to 160c/320f. Grease and line the base only of a 23cm / 9" tart tin
With a loose base. (or a spring form pan of a similar size)
Roughly break up biscuits and place in a food processor, then whizz until fine crumbs form. Or do this step with a zip lock bag and rolling pin.
Pour crumbs into a bowl, add melted butter, mix until it looks like wet sand and no dry crumbs remain. Pour into tin, then press firmly into base and sides – use something flat like a cup to assist
Transfer to tray (safe handling), place in oven and bake for 10 minutes, then remove and cool slightly (5 minutes in fridge).Place cream and chocolate in a heat proof bowl.
Microwave in 2 x 30 second bursts, stirring in between, until smooth.