Preheat your oven to 325°F (163°C) and grease a mini muffin pan with non-stick spray or use mini parchment liners.
In a microwave-safe bowl, heat the butter and chocolate chips in 30-second intervals, stirring each time until the mixture is smooth.
Mix the sugar into the warm chocolate mixture, stirring until it cools down and becomes uniform, about 2 minutes. Then, stir in the eggs.
Fold in the flour, cocoa powder, and salt into the wet mixture with a spatula, mixing just until you can’t see any flour.
Use a small cookie scoop or spoon to fill each muffin well about halfway with the brownie batter, and place a mini peanut butter cup in the center of each.
Bake for 9-12 minutes, or until the edges look set and slightly dry. Allow the brownies to cool in the pan for about 10 minutes.
Remove the brownies from the pan, twisting and pulling them out if the pan is well-greased, or gently using a knife if they stick.