Preheat oven to 350°F (175°C)
Make the Crust: Mix crushed graham crackers, sugar, and melted butter. Press firmly into a parchment-lined 9x9-inch pan.
Bake the crust for 8-10 minutes, or until lightly golden.
Make Ganache: In a microwave-safe bowl, heat the heavy cream and chocolate chips in 30 second increments, stirring in between each round in the microwave. Stir until the chocolate has dissolved into the cream and everything is smooth and glossy.
Assemble Layers: Pour the chocolate ganache over the baked graham cracker crust, spreading it evenly. Let it cool for 5-10 minutes at room temperature to slightly set.
Top with Marshmallows: Sprinkle the marshmallows in an even layer on top of the ganache.
Broil for Gooey Marshmallows: Set your oven to broil and place the pan on the middle rack. Broil on low for 3-4 minutes, watching carefully, until the marshmallows are golden and gooey. Actual time being broiled may depend on your oven.
Cool and Eat: Allow your s'mores bars to cool before digging in. You can eat them warm, or set them in the fridge to firm up a bit. If you let them cool in the fridge, they're easy to cut into bars.
Store: Store leftovers in the pan you baked them in, covered in the fridge. To serve fresh s'mores bars, slice a bar off and microwave it on a plate for 5-10 seconds to warm it up.