To Make The Brownies:

1. Position a rack in the lower third of the oven and preheat the oven to 325°F. Line the bottom and sides of a 9×13-inch baking pan with parchment paper. Lightly grease with cooking spray and set aside.
2. Combine the butter, sugar, cocoa, and salt in a medium microwave-safe bowl. Microwave for one minute intervals, stirring in between, until the butter is melted and the mixture is smooth. Set the mixture aside until it cools to room temperature. (You can pop it in the fridge to speed up this process.)
3. Stir in the vanilla. Add the eggs one at a time, stirring vigorously after each one. When the batter looks thick, shiny, and well blended, add the flour and stir until you cannot see it any longer, then beat vigorously for 40 strokes with the wooden spoon or a rubber spatula. Spread evenly in the lined pan.
4. Bake until a toothpick inserted into the center emerges slightly moist with batter, 20 to 25 minutes. Let pan cool completely on a rack.
5. Then spread the prepared peanut butter fudge (see instructions below) in a single layer on top of the brownies, smoothing with a rubber spatula. Sprinkle with the 1/2 cup of mini chocolate chips.
6. Cover and refrigerate for at least 1 hour, or until the fudge has set. Then carefully lift the peanut butter fudge brownies out of the pan by pulling on the parchment paper, and set on a cutting board. Cut the brownies into squares and serve.

To Make The Peanut Butter Fudge:

1. Bring butter and peanut butter to a boil in medium saucepan over medium-high heat, stirring occasionally until combined and smooth. Remove from heat and stir in salt and vanilla until combined. Gradually add in powdered sugar, stirring until combined. Then immediately add fudge to brownies. (See above.)