Preheat oven to 350F. Line an 8x8-inch baking pan with parchment paper or foil. Spray it with cooking spray.
In a mixing bowl, sift together flour, 2 tablespoons cocoa powder, and 1/2 teaspoon salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat together 1/2 cup butter, sugar, and 3/4 teaspoons vanilla until light and fluffy. Add the eggs one at a time, mixing sure each is fully incorporated. Mix dry ingredients into batter just until combined. Pour batter into prepared pan and spread evenly. Sprinkle with chocolate chips. Bake in preheated oven for 22-25 minutes or until a toothpick inserted into the center comes out clean.
When brownies are done baking, combine peanut butter and oil in a microwave safe bowl and microwave on high for 20 seconds to soften. Pour over hot brownie and spread it to the sides. Sprinkle with marshmallows. Return to oven to bake for 3 more minutes, or until the marshmallows puff. Remove from oven and top with toasted pecans.
To make frosting, combine confectioners sugar and salt in the bowl of a stand mixer fitted with a paddle attachment. In a saucepan, melt 4 tablespoons butter over medium heat. When butter is melted, add the 2 Tablespoons of cocoa powder and 1/4 cup of the milk and heat until the mixture just comes to boil. Pour this mixture into confectioners sugar/salt mixture and beat on moderately high speed until smooth and glossy. If frosting is not pourable at this point, add remaining 2 tablespoons of milk Lastly, beat in 1/4 teaspoon vanilla, then pour evenly over puffed marshmallows and pecans.