Line an 8x8 pan with parchment paper, leaving some overhang for easy removal.
Arrange a layer of graham crackers on the bottom of the pan, breaking them as necessary to fit snugly.
Melt the chocolate chips in a microwave-safe bowl in 30-second intervals, stirring between each, until smooth. Drizzle half of the melted chocolate over the graham crackers, spreading it out evenly.
In a mixing bowl, combine Cool Whip, Greek yogurt, marshmallow fluff, and vanilla extract. Stir until well combined and creamy. Fold in the mini marshmallows (and mini chocolate chips, if using) to the mixture.
Spread the filling mixture evenly over the chocolate-drizzled graham cracker base. Place another layer of graham crackers on top of the filling, pressing them gently to adhere. Drizzle the remaining melted chocolate over the top layer of graham crackers.
Cover the pan with plastic wrap or foil and place in the freezer for at least 4 hours, or until the bars are fully set. Once frozen, lift the bars out of the pan using the parchment paper overhang. Cut into squares and serve immediately.