- 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.