Line 9 x 13-in. (23 x 32-cm) baking pan with parchment paper, with overhang. Pour melted milk chocolate into prepared pan and spread evenly. Sprinkle mini marshmallows, dark chocolate and graham crumbs over top. Gently press toppings into melted chocolate.
Refrigerate 1 hr., or until chocolate is set. Remove from pan and peel off parchment paper. Break bark into pieces. Store in airtight container for up to 1 week.