Prepare the pan. Line a 9x9-inch baking pan with parchment paper and lightly coat with nonstick spray. For thinner bars, use a 9x13-inch pan instead.
Melt the chocolate mixture. Add the chocolate chips and butter to a large microwave-safe bowl. Microwave in 30-second intervals, stirring well after each, until smooth and completely melted.
Add the sweetened condensed milk. Stir until the mixture is thick, glossy, and silky smooth. This is what gives the candy its rich, fudge-like consistency.
Fold in the mix-ins. Add the peanuts and mini marshmallows, then gently stir until everything is evenly coated in chocolate. Work fairly quickly because the mixture will begin to set as it cools.
Spread into the pan. Transfer the mixture to the prepared pan and smooth it into an even layer. If desired, sprinkle a little flaky sea salt over the top for an extra sweet-and-salty finish.
Chill until firm. Refrigerate for about 2 hours, or until the rocky road candy is completely set.
Slice and enjoy. Lift the candy out using the parchment paper, cut into squares, and try not to eat half the pan while "testing" a piece.