Preheat oven to 325 degrees F (160 degrees C). Spray the bottom and sides of an 8-inch square pan with Vegetable Oil Cooking Spray (to taste). Line 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 Unsalted Butter (1/2 cup), Granulated Sugar (1 cup), Unsweetened Cocoa Powder (3/4 cup), and Salt (1/4 teaspoon).
Heat in the Panasonic Microwave Oven 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 (1 teaspoon) and add the Large Egg (2), one at a time, stirring after each addition. Add the All-Purpose Flour (1/2 cup) and stir until just combined.
Add the Peanut Butter Chips (1/2 cup), Chocolate Chips (1/2 cup), Mini Marshmallows (1/2 cup) and Salted Peanuts (1/2 cup). Fold into the batter. Pour into the baking pan and smooth the top with a spatula.
Sprinkle the remaining 1 Tbsp of Peanut Butter Chips, Chocolate Chips, Mini Marshmallows, and Salted Peanuts on top of the brownies. Lightly press them into the batter with your fingertips.
Bake for 25 to 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 to 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. Store in an airtight container. Cooking results may vary depending on microwave oven used.