Line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
In a microwave-safe bowl, combine the chopped semisweet chocolate, milk chocolate, and butter. Microwave in 30-second intervals, stirring after each, until the mixture is smooth and fully melted.
Remove the bowl from the microwave and let the chocolate mixture cool slightly for 2-3 minutes to avoid melting the marshmallows.
Gently fold in the mini marshmallows, chopped peanuts, and graham cracker pieces until evenly distributed throughout the chocolate.
Pour the mixture into the prepared pan, spreading it evenly with a spatula.
Refrigerate the pan for at least 2 hours, or until the rocky road is fully set and firm.
Once set, lift the rocky road out of the pan using the parchment paper overhang and cut it into 16 even squares with a sharp knife.