01 - Line an 8-inch (20 cm) square baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium bowl, mix graham cracker crumbs, melted butter, and sugar until combined. Press evenly into the bottom of the prepared pan to form a firm base.
03 - In a microwave-safe bowl, combine 150 g mini marshmallows and 30 g butter. Microwave in 20-second bursts, stirring until melted and smooth. Spread evenly over the graham base. Let cool for 5 minutes.
04 - In another microwave-safe bowl, combine chocolate chips and heavy cream. Microwave in 20-second intervals, stirring until melted and smooth. Pour over the marshmallow layer and spread evenly.
05 - Immediately sprinkle the remaining mini marshmallows, graham cracker pieces, and optional chocolate chips over the top. Gently press toppings into the chocolate layer.
06 - Refrigerate for at least 1 hour or until set. Lift out of the pan using the parchment overhang. Cut into bars and serve.