Preheat your oven to 350°F (175°C) and lightly grease a 9x13-inch baking pan or line it with parchment paper for easy removal.

In a medium bowl, combine the graham cracker crumbs, melted butter, and granulated sugar. Mix until evenly combined, then press the mixture firmly into the bottom of the prepared baking pan to form the crust.

Sprinkle the milk chocolate chips evenly over the graham cracker crust.

Next, sprinkle the mini marshmallows evenly over the chocolate layer.

Pour the sweetened condensed milk evenly over the marshmallows and chocolate, making sure to cover as much of the surface as possible.

Bake the s'more bars in the preheated oven for 20-25 minutes, or until the edges are golden brown and the marshmallows are puffed and slightly toasted.

Remove the pan from the oven and allow the bars to cool completely at room temperature, then transfer to the refrigerator to chill for at least 1 hour to firm up.

Once chilled, slice the s'more bars into squares and serve. Store any leftovers in an airtight container in the refrigerator.