Toast the marshmallows: Preheat oven to 250°C/480°F/Gas Mark 9 (or switch on the broiler setting). Line a baking tray with parchment paper (nonstick cooking paper), and spread out the mini marshmallows on top. Place in the oven on the top rack for 1 minute or until toasted and golden brown. Transfer the tray to a wire cooling rack and allow to cool completely. Set aside until needed.
Make the chocolate shards: Melt the unsalted butter and milk chocolate chips in a microwave safe measuring cup (or bowl) on medium power for 30-40 seconds, stirring in between until smooth. Pour the chocolate mixture on a parchment paper lined baking tray. Use an offset or silicone spatula (or the back of a large spoon) to smooth out the chocolate in a thin layer. Place in the freezer for 15-20 minutes, or until firm. Transfer to a cutting board and cut the chocolate into shards of your desired size. Store in the freezer until needed.
Make the whipped cream: Pour the heavy whipping cream into a large mixing bowl. Using an electric hand mixer (or stand mixer with the whisk attachment), beat the cream starting on low speed and increasing the speed as the cream thickens, until stiff peaks form – about 4-12 minutes. (Note: Time for beating the cream is approximate and will vary depending on several factors such as environmental conditions, how cold the cream is, etc.)
Beat the sweetened condensed milk and vanilla: Combine the sweetened condensed milk and vanilla extract in another large mixing bowl. Beat with the electric hand mixer (no need to wash or clean the beaters after making the whipped cream) on medium speed for 1 minute until combined well.
Fold: Starting with one big dollop, gently fold the whipped cream into the sweetened condensed milk mixture to lighten it up a little. Then add the rest of the whipped cream and gently fold until combined well. The mixture should be slightly thick and creamy. Now gently fold in the toasted mini marshmallows, chocolate shards, and crushed Digestive biscuits until combined.
Freeze: Transfer to ice cream paper cartons, or other type of sealable freezer safe airtight container such as insulated ice cream tubs, or a bread loaf pan. Cover the surface with parchment paper or cling wrap, making sure that it touches the ice cream. Seal the container(s) with its lid, then freeze for 4-6 hours or until solid. (If using a bread loaf pan, simply cover well with the parchment paper or cling wrap to ensure ice crystals don’t form during freezing.)
Serve: Once the ice cream is solid, place on the counter at room temperature for 5-10 minutes until slightly softened. Scoop into bowls and drizzle hot fudge sauce on top if desired, then enjoy!
Storing: Store in a sealed airtight freezer safe container in the freezer for 3-4 weeks. If storing in a bread loaf pan for more than a few days, transfer the ice cream to a freezer safe sealed airtight container so that it can stay fresh longer and ice crystals don’t start to form.
To make it gluten-free: Use marshmallows made with corn starch instead of wheat starch, and gluten-free Digestive biscuits (available at Tesco if you are in the UK) or graham crackers (Schär, Kinnikinnick, and Pamela’s). Make sure that the chocolate chips you’re using are gluten-free too.
To make it vegan: Use vegan mini marshmallows such as Dandies, dairy-free butter, dairy-free chocolate chips, vegan sweetened condensed milk (Carnation has one that isn’t coconut milk/cream based), and a dairy-free whipping cream to make it vegan. Make sure that the Digestive biscuits or graham crackers you’re using are dairy-free too. Most are, but it’s best to double check the ingredients label to be sure.