Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.

In a large mixing bowl, combine graham cracker crumbs, melted butter, and granulated sugar. Mix until well combined and moist.

Press the mixture evenly into the bottom of the prepared baking pan. For a slightly sturdier crust, you can bake it at 350°F (175°C) for 8-10 minutes, then let it cool completely. Otherwise, simply chill for 15 minutes.

In a large mixing bowl, using an electric mixer, beat together creamy peanut butter and softened unsalted butter until smooth.

Gradually add the sifted powdered sugar, beating until fully incorporated and creamy. Add the vanilla extract and mix until just combined.

Evenly spread the peanut butter mixture over the cooled or chilled graham cracker crust.

In a medium saucepan over low heat, melt the mini marshmallows and 2 tablespoons of unsalted butter, stirring constantly until smooth and gooey. Be careful not to scorch them. Alternatively, you can melt them in a microwave-safe bowl in 30-second intervals, stirring in between, until smooth.

Quickly and carefully spread the melted marshmallow mixture evenly over the peanut butter layer. It will be very sticky. You can lightly grease your spatula with a little butter or cooking spray to help with spreading.

In a microwave-safe bowl, combine semi-sweet chocolate chips and heavy cream. Microwave for 30 seconds, stir, then microwave in 15-second intervals, stirring after each, until the chocolate is completely melted and smooth. Alternatively, melt over a double boiler.

Pour the melted chocolate ganache over the marshmallow layer, spreading it gently and evenly to cover the entire surface.

Refrigerate the bars for at least 2 hours, or until the chocolate topping is firm and the layers are set.

Once firm, use the parchment paper overhang to lift the bars out of the pan. Place on a cutting board and cut into 24 squares (or desired size).

Store leftover bars in an airtight container in the refrigerator for up to 5 days.