Nonstick cooking spray, for the pan, 1/2 cup (1 stick) unsalted butter, 110.5-ounce bag miniature marshmallows, 1 cup sugar, 1 cup creamy peanut butter, 1 cup heavy cream, 1/4 teaspoon kosher salt, 16 ounces semisweet chocolate, chopped, 1/4 cup coarsely chopped peanuts
Coat an 8-inch square baking pan with nonstick cooking spray and line with a piece of parchment or wax paper, leaving an overhang on two sides.
In a medium saucepan, combine the butter, marshmallows, sugar, peanut butter, cream, and salt. Cook over medium heat, stirring frequently, until the marshmallows have melted and the mixture is smooth, 8 to 10 minutes. Add the chocolate and stir until melted.
Pour the mixture into the prepared pan and sprinkle with the peanuts. Let cool for 30 minutes, then refrigerate until set, at least 3 hours and up to overnight. Use the paper to lift the fudge out of the pan; cut into 36 squares.