3 tbsp. unsalted butter, plus more for greasing the pan
1 cup graham cracker crumbs
¾ cup sweetened condensed milk
1 1/3 cups semisweet chocolate chips
1½ tsp. vanilla extract
Pinch of salt
1 cup mini marshmallows
2 whole graham crackers
Line an 8 x 8-inch baking dish with aluminum foil. Lightly butter the foil. Place the butter in a heatproof bowl and microwave until melted. Mix in the graham cracker crumbs and stir with a fork until the crumbs are evenly moistened. Pour the crumb mixture into the bottom of the prepared pan and press firmly into an even layer on the bottom of the pan. (I like to use the bottom of a ramekin to do this.)
In a medium saucepan over medium heat, combine the condensed milk and chocolate chips. Heat, stirring occasionally, until the chocolate is completely melted and the mixture is smooth. Off the heat, stir in the vanilla and salt. Pour the chocolate mixture over the graham cracker base. Working quickly, spread the chocolate into an even layer and smooth with a spatula. Sprinkle the marshmallows over the top of the chocolate. Gently press down so that the marshmallows become partly embedded in the chocolate. Break the whole graham crackers into small pieces and stick into the chocolate mixture between the marshmallows.
Cover and refrigerate the bars until firm, about 4 hours. Use the foil to lift the bars out of the pan and transfer to a cutting board. Use a large sharp knife to cut into squares or rectangles. Refrigerate until ready to serve.