Preheat the oven to 350°F. Line an 8 x 8 baking pan with aluminum foil.
Add the butter and chocolate to a bowl and melt together in the microwave at 50% power for 30 second increments, stirring between each increment, until the butter and chocolate is melted. Mix well together. Set aside to cool slightly.
In another bowl, mix together the flour, baking powder, and salt. Mix together.
Add the sugar, eggs, and vanilla to the butter mixture and mix until just combined. Do not over whip!
Stir the flour mixture into the wet ingredients.
Add half the batter to the baking pan and then top with the peanut butter cups. Top with the remaining batter.
Add dollops of the peanut butter over the batter, and use a knife to create swirls of the peanut butter around the batter.
Bake for 30-35 minutes, or until a toothpick comes out with a few crumbs attached.