Line your baking pan with parchment paper and lightly spray it with cooking spray to prevent sticking.
Combine the graham cracker crumbs and melted butter in a bowl, stirring until the mixture is evenly coated and holds together when pressed.
Press the crumb mixture firmly into the bottom of the prepared pan, creating an even crust. Bake at 350°F (175°C) for about 8-10 minutes until lightly golden and fragrant.
In a saucepan over low heat, melt the mini marshmallows, stirring constantly until smooth and gooey, about 5 minutes. Add vanilla extract and mix well.
Pour the melted marshmallow mixture over the baked crust and spread evenly with a spatula. Allow it to set and cool for about 10-15 minutes until slightly firm.
Meanwhile, melt the chocolate chips in a microwave-safe bowl in 30-second intervals, stirring between each, until smooth and glossy.
Pour the melted chocolate over the marshmallow layer, using a spatula to smooth it out before it begins to set. Wait a few minutes until the top is shiny and firm.
Refrigerate the assembled bars for at least 2 hours or until completely chilled and set for clean slicing.
Remove the bars from the refrigerator, lift them out using the parchment paper, and cut into squares with a sharp knife. The crackly top and dense interior will be revealed as you slice.
Serve these rich, chewy bars with a glass of milk or your favorite hot beverage, enjoying the sticky, gooey, chocolatey goodness.
