Add heavy cream to a large mixing bowl and beat with an electric mixer until it forms stiff peaks.
Fold the sweetened condensed milk and vanilla into the whipped cream.
Gently stir in the marshmallows, graham crackers, and chocolate bar into the mixture, reserving some for sprinkling over the top of the ice cream, if desired.
Spread mixture into a freezer safe dish such as a metal loaf pan.
Cover tightly and freeze for 4 hours or overnight.
To give this a toasty marshmallow vibe, spread the marshmallows onto a baking sheet and place under the broiler until browned on top. Stir into the ice cream mixture as directed in step 3.Serve with a drizzle of hot fudge and extra graham crackers, if desired.