Place a 2-litre freezer-safe container (I use a reusable plastic ice cream container, but you can also use a metal loaf tin) in the freezer until ready to use.
Place the chopped chocolate in a small bowl. Heat the cream in a small saucepan over medium heat until just before boiling (do not let boil). Pour the cream over the chocolate and stir until completely melted and smooth. Refrigerate for 20 minutes to cool.
Meanwhile, heat marshmallows and milk in a medium saucepan over low heat and stir until melted. Remove from heat and set aside.
Transfer the ice cream to the bowl of a stand mixer and beat on low speed until softened (but not melted).
Working quickly, add the marshmallow mixture, chocolate ganache and biscuit crumbs to the ice cream and fold gently with a big wooden spoon or spatula a few times to get a swirly look. Make sure it's only just swirled through and that you still have globs of marshmallow and ganache - you don't want to completely mix these in.
Transfer the ice cream to the pre-chilled container and freezer for 2-4 hours or until set.
Will keep in the freezer for 4-5 days.
Use graham crackers if they are available where you live. In Australia, we have to use alternatives such as malt biscuits!