Blitz the biscuits into crumbs using a food processor, or you can put them in a bowl or freezer bag and bash them up gently with the end of a rolling pin
Place the biscuit crumbs into a bowl. Add the melted butter and mix it in
Press the biscuit mixture into the base and sides of a 9" loose bottomed tart tin. An American cup measure, or a shot glass, can help you to evenly press it around the sides and base
Put the biscuit base in the fridge for 30 minutes
Spread the marshmallow fluff out onto the base of the tart
To make the chocolate ganache, warm the double cream up in a pan on a low heat, give it a stir reguarly
Take the cream off the heat when it starts to steam. Add the chocolate to the cream and leave it for a couple of minutes, then stir until it all melts together. Then stir in the pinch of salt
Pour the ganache into the biscuit base over the marshmallow fluff. Gently spread it out and smooth the top using a small palatte knife. You can also tap the tin very gently on the work surface to even out the ganache and remove any air bubbles
Leave in the fridge for at least 4 hours, or ideally overnight, to set
Remove the tart from the tin, put the cocoa powder into a fine sieve and dust it all over the top of the tart
Whip up the double cream with the icing sugar and vanilla extract and pipe it around the edges of the tart
Put the marshmallows onto a plate and use a kitchen blowtorch to toast them. Or you can pop them under the grill briefly
Place the toasted marshmallows onto the whipped cream
Serve immediately. Store leftovers in the fridge and eat within 3 days