Preheat the oven to 300°F. Line a 9-by-13-inch baking pan with nonstick foil (with the nonstick side facing up). Drape some of the foil over the sides of the pan. Lightly coat the foil with nonstick cooking spray.

In a large microwave-safe bowl, melt the butter in a microwave. Add the sugar, vanilla and eggs and mix thoroughly. 

In another bowl, sift together the flour, dark cocoa, baking powder and salt. Add to the bowl with the butter mixture and stir until just incorporated. Pour the batter into the prepared pan.

Unwrap the peanut butter cups. Gently press the peanuts butter cups into the batter, forming 6 rows across and 4 rows down. The baked brownies will be cut into twenty-four 2-inch squares, so space the peanut butter cups accordingly.

Bake until the brownie springs back when lightly touched, about 25 minutes. Transfer the pan to a wire rack and let cool for 30 minutes.

Run a knife along the inside edge of the pan. Lift out the brownies by pulling up on the overhanging foil.

When the brownies are cool, cut into twenty-four 2-inch squares and top each with a small dollop of chocolate ganache. 

Place the chocolate chips in a heatproof bowl.

In a microwave-safe bowl, combine the cream and butter and heat in the microwave in 30-second increments, stirring after each one, until the chocolate is melted. Pour the cream mixture over the chocolate chips and stir until they have melted. Let the ganache cool until it is the consistency of icing.