Preheat oven: Set your oven to 350°F (175°C). Line a 9×13-inch baking dish with parchment paper or spray with nonstick cooking spray for easy removal and cleanup.
Make the crust: In a medium bowl, mix together the melted butter, graham cracker crumbs, sugar, and salt. Stir until the mixture resembles wet sand and all the crumbs are evenly moistened.
Press and layer: Press the crust mixture firmly and evenly into the bottom of the prepared baking dish. Pour the sweetened condensed milk evenly over the crust layer. Sprinkle the milk chocolate chips evenly over the top of the milk layer.
Bake the base: Bake in the preheated oven for 20–25 minutes, or until the edges start to bubble and brown and the chocolate chips are glossy and slightly melted.
Add marshmallows: Carefully remove the pan from the oven and sprinkle the mini marshmallows evenly over the top. Return the pan to the oven and bake for another 5–7 minutes, or until the marshmallows are golden brown and puffed.
Top and cool: Immediately sprinkle the crushed graham crackers over the top of the toasted marshmallows. Let the bars cool completely at room temperature.
