In a large mixing bowl, combine the graham cracker crumbs, rolled oats, and cocoa powder. Mix well to combine all the dry ingredients.
In a microwave-safe bowl, heat the peanut butter and honey (or maple syrup) together for about 20-30 seconds until warm and more easily mixable. Remove from the microwave and stir in the vanilla extract.
Pour the peanut butter mixture into the dry ingredients and mix until everything is well-coated. The mixture should be sticky but not too wet. If it's too dry, add a bit more honey or peanut butter.
Gently fold in the mini marshmallows and chocolate chips until evenly distributed throughout the dough.
Line a baking dish (around 8x8 inches works well) with parchment paper for easy removal. Spread the cookie mixture into the dish evenly, pressing down firmly to create a solid layer.
Chill the cookies in the refrigerator for at least 30 minutes until set.
Once set, lift the cookies out of the dish using the parchment paper and cut into squares or bars.
Notes
Arrange the squares on a decorative plate and drizzle with melted chocolate for an extra treat. Sprinkle some extra graham cracker crumbs and mini marshmallows on top for a fun S'mores touch!