Line a 450g (1 lb) loaf tin with cling film and set aside. Set a heatproof bowl over a pot of gently simmering water. Add in the chocolate and butter and allow to melt, stirring now and then until it’s smooth. Remove the bowl from the heat and mix in the espresso powder and salt, then pour in the condensed milk, stirring well to combine. Add in the biscuits and stir until they’re evenly distributed in the chocolate. Pour the chocolate biscuit mixture into the lined loaf tin, pressing it down evenly and firmly with a spatula. Smooth the top with the spatula, then place in the fridge for about 3 hours, or overnight, until set. Cut into thin slices or small cubes to serve. Wrapped tightly in cling film, this will keep for a couple weeks in the fridge.