Preheat oven to 325 degrees. Spray the bottom and sides of an 8" square pan with vegetable spray. Line the the bottom of the pan with parchment paper, leaving an overhang on either side to assist in lifting the brownies from the pan. Set aside.
Into a large glass or other microwave safe bowl, add the butter, granulated sugar, cocoa powder and salt. Heat in the microwave in 30 second increments until the butter is melted. Stir until the mixture is completely combined. Set aside to cool while you assemble the other ingredients.
Stir in the vanilla extract and add the eggs, one at a time, stirring after each addition. Add the flour and stir until just combined. Add the half cup of peanut butter chips, chocolate chips, miniature marshmallows and peanuts. Fold into the batter. Pour into the baking pan and smooth the top with a spatula. Sprinkle the remaining 1 tablespoon of the chips marshmallows and peanuts on top of the brownies. Lightly press them into the batter with your fingertips.
Bake for 25-30 minutes and check for doneness with a cake tester or toothpick. If it comes out with just a few crumbs, the brownies are done. If you cook them too long, they'll be more cake-y than fudgy. If the batter is still liquidy, cook for an additional 3-5 minutes.
Let the brownies cool to room temperature. Run a knife around the edges and gently lift them from the pan onto a cutting board. Use a long, sharp knife to cut them into squares.