Mega Peanut Butter Swirl Brownies

 cup (16 Tbsp; 2 sticks; 230g) unsalted butter, divided

(113g) semi-sweet chocolate bar, coarsely chopped1

granulated sugar

large eggs, at room temperature

pure vanilla extract, divided

unsweetened natural or dutch-process cocoa powder2

all-purpose flour (spoon & leveled)

salt

and 1/2 cups chopped peanut butter cups

creamy peanut butter3

confectioners' sugar