Pour both amounts of milk into a saucepan, place over medium heat, and bring milk almost to a simmer, stirring often. Add marshmallows and chocolate candy bars. Stir until marshmallows and chocolate melt and the mixture is smooth. Set aside to cool.
Whip cream in a large bowl until fluffy and cream holds stiff peaks when beaters are lifted straight up, about 5 minutes. Gently fold cooled marshmallow mixture into whipped cream.
Combine graham cracker crumbs, sugar, and butter until crust mixture is evenly moist. Press mixture into the bottom of a 9x13-inch dish. Spread marshmallow filling evenly over the crumb crust. Refrigerate until set, about 2 hours.