Preheat your oven to 350°F. Grease a 9x9-inch baking pan or line it with parchment paper for easy removal.
In a medium bowl, combine the graham cracker crumbs and melted butter until well mixed. Press the mixture firmly into the bottom of the prepared baking pan to form a crust.
In a separate bowl, mix the mini marshmallows, chocolate chips, sweetened condensed milk, and vanilla extract until combined. Pour this mixture over the graham cracker crust and spread it evenly.
Sprinkle the additional mini marshmallows on top of the chocolate mixture.
Bake in the preheated oven for 25-30 minutes, or until the marshmallows are golden brown and the mixture is bubbly.
Remove from the oven and let it cool in the pan for at least 15 minutes before slicing into bars.
Serve warm or at room temperature.