Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish or line it with parchment paper for easy removal.
In a medium bowl, combine the graham cracker crumbs, cocoa powder, and granulated sugar. Mix well.
Pour in the melted butter and vanilla extract, stirring until the mixture is well combined and resembles wet sand.
Press the mixture evenly into the bottom of the prepared baking dish to form a crust.
Bake the crust in the preheated oven for 10 minutes. Remove from the oven and let it cool slightly.
Evenly sprinkle the chocolate chips over the crust, followed by the mini marshmallows. If using, drizzle the peanut butter over the top.
Return the dish to the oven and bake for an additional 5-7 minutes, or until the marshmallows are golden and the chocolate is melted.
Allow the sliders to cool for about 15 minutes before slicing into squares. Serve warm or at room temperature.