Crush the graham crackers into fine crumbs using a food processor or by sealing them in a zip-top bag and rolling with a rolling pin. Pour the crumbs into a mixing bowl, then add the melted butter and stir until the mixture resembles damp sand.
Press the graham cracker mixture evenly into the bottom of a greased 9x13 inch baking pan, creating a firm crust. Use the back of a spatula or your fingers to press it down firmly, then refrigerate for 10 minutes to set.
Place the chocolate chips in a small saucepan over low heat or melt them in short bursts in the microwave, stirring until smooth. Mix in the vanilla extract to enhance the flavor, then pour the melted chocolate evenly over the chilled crust.
Spread the chocolate layer carefully with a spatula to smooth out the surface. Allow it to cool slightly so it thickens and sets a bit while you prepare the marshmallows.
Arrange the marshmallows evenly over the chocolate layer, covering the surface completely. Use your fingers or a spoon to distribute them so they are packed closely together.
Preheat your broiler or oven with the broil setting. Place the pan on the top rack and toast the marshmallows for 1-2 minutes, watching carefully so they don’t burn. You want them to turn golden brown and puff up beautifully.
Once the marshmallows are toasted to your liking, remove the pan from the oven and let it cool for a few minutes. The marshmallow topping will be sticky and glossy, with a toasted aroma filling the kitchen.
Cut the bars into squares using a sharp knife, warming the blade with hot water and wiping dry between cuts for cleaner slices. Serve immediately for the best gooey texture or let set for a few minutes for firmer bars.
