Preheat oven to 350°F. Grease and line a 9″x9″ pan with parchment paper and set aside.
In a large microwave-safe bowl, combine butter and sugar. Microwave for 30 seconds, then remove and stir with a whisk. Repeat process 2 more times. Though granules of sugar will remain, the goal is to get the mixture to a glossy finish.
Allow mixture to cool slightly, then add in eggs one at a time, whisking in between each addition. Whisk until the mixture lightens in color slightly.
Add in cocoa powder, salt, and vanilla extract. Fold in flour until just combined.
Pour mixture into prepared baking pan and bake for roughly 45 minutes.Stick a knife inside the center. Once it comes out relatively clean, take the brownies out of the oven. Sprinkle the tops with the marshmallows, chocolate chips and chopped peanuts. Turn your oven to a broil to get the marshmallows to crisp up. Place the brownies back in the oven and cook for another 2 to 3 minutes, until the marshmallows are toasted. Keep a close eye on the marshmallows, rotating the pan to ensure even toasting.
Take the brownies out of the oven and allow to cool before slicing. Enjoy!