Preheat oven to 350ºF and line an 8? square pan with foil. Grease foil with nonstick spray.
In a small saucepan, combine the butter and chocolate and heat over medium-low heat, stirring occasionally, until smoothly melted and combined. Set aside to cool slightly.
Meanwhile, in a medium bowl, whisk together the cocoa powder, cornstarch and salt.
In the bowl of a stand mixer fitted with the wire whip attachment, or using a bowl and handheld electric mixer, combine eggs and sugar and beat on medium-high speed until mixture is light in color and ribbons down from the beater when lifted. Wipe down bowl and beater as needed. Beat in vanilla extract.
Add the dry ingredients to the wet and mix in on low speed just until partially combined. Finish folding together by hand until all ingredients are evenly incorporated. Fold in peanut butter cups just until dispersed.
Transfer batter to prepared pan and smooth out in an even layer. Tap pan firmly on counter a few times to help settle and remove any air bubbles.
Bake brownies in preheated oven for 25-30 minutes, until a toothpick inserted into the center comes out clean or with only a few moist crumbs, and edges are slightly pulling away from sides of pan.
Cool in pan on a wire rack until completely cool, and then use foil to lift from pan and slice. For even cleaner slicing, transfer to fridge for 30-60 minutes before slicing.
Store in an airtight container at room temperature.