Preheat the oven to 325ºF. Line a 9"x13" baking pan with parchment paper and spray with non-stick spray.

In a food processor, pulse the graham crackers until they are finely chopped.

Add the melted butter and brown sugar to the food processor and pulse until the mixture resembles gravel.

Pour the mixture into the prepared baking pan and press down firmly to form a crust.

Bake on the center rack until golden, about 10 minutes.

Remove from the oven.

Set up a heat-proof bowl over a pot of simmering water. Make sure it doesn't boil.

Place the chocolate chips and sweetened condensed milk in the bowl and stir constantly until fully melted.

Pour it over the crust. Quickly even it out with the back of a spoon. Allow to cool for at least 15 minutes

Scatter the marshmallows over the top of the chocolate.

Heat the oven on broil with the rack near the top of the oven.

Place the baking pan under the broiler until the marshmallows are toasted, about 2 minutes.

Remove from the oven and cool for about 10 minutes.

Remove the bars from the pan with the parchment paper. Cut and enjoy.