Prep: Preheat oven to 325°F. Line the bottom and sides of a 9x13-inch baking pan with parchment paper & lightly coat with nonstick spray.
Brownie batter: Combine the butter, sugar, cocoa, and salt in a medium microwave-safe bowl. Microwave for 30-second intervals, stirring in between, until the butter is melted and the mixture is smooth (I cut my butter and only had to heat it for 30 seconds). Set the mixture aside until it cools to room temperature.
Add the vanilla extract, then the eggs one at a time, stirring vigorously after each. Add the flour and stir until combined, then beat vigorously for 40 strokes with the wooden spoon or a rubber spatula. Spread evenly in the lined pan. (Batter will be quite thick.)
Bake: Bake until a toothpick inserted into the center emerges slightly moist with batter, 20-23 minutes. Let pan cool completely on a rack.
Peanut Butter Fudge Topping: 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.
Assemble & Chill: Immediately pour fudge on brownies. Spread the prepared peanut butter fudge Sprinkle with the ½ cup of mini chocolate chips/candy bar or chocolate chunks.
Refrigerate until the fudge sets (about an hour). 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. Enjoy!