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.
For roasted marshmallows, lightly toast some marshmallows under broiler on foil-lined baking pan or use blowtorch to toast before sprinkling on melted chocolate.
For nutty toffee s’mores bark, fold chopped nuts and toffee bits into melted chocolate.