For the brownies, cut the butter into pieces and place it in a microwave-safe bowl with the semisweet chocolate. Place the bowl in the microwave and melt the contents in increments, stirring in between. Let it cool to room temperature.

Heat oven to 350 degrees Fahrenheit. Line 18 to 20 cups of a standard muffin tin with cupcake liners.

Combine the sugars in a large mixing bowl. Add the eggs and sugar and mix with an electric mixer until well blended. Blend in the vanilla extract. Add cooled chocolate (it should still be liquid) and mix on medium speed just until evenly blended.

Stir together the flour and salt in a separate bowl, then stir them into the chocolate mixture, about half at a time, until no streaks of flour remain.

Prepare the peanut butter filling by adding the peanut butter, cream cheese, egg, and sugar to the bowl of an electric mixer and creaming them together until evenly blended. The texture should be like whipped peanut butter.

Fill each muffin cup about halfway with brownie batter, then use a spoon to create a small indent in the center of the batter for the peanut butter filling to sit. Spoon about 2 tsp. of peanut butter filling into the center, pushing it down slightly into the batter. Cover the filling with another spoonful of batter, making sure that the cups are no more than two thirds to three quarters full.

Bake for approximately 17 to 20 minutes, just until well risen and cracked on top. Then transfer pans to wire rack and let cool for 15 minutes. Remove the brownies from the pan and place them directly on the wire rack to finish cooling (the centers will sink as they cool).

Let them cool to room temperature, then prepare the frosting by creaming together butter, powdered sugar, vanilla, and cocoa powder in the bowl of an electric mixer. Add milk a tablespoon at a time, mixing in between, to thin to desired consistency. Mix it for at least 5 minutes on high speed to make a creamy, delicious frosting! Place a dollop of frosting in the center of each brownie cup where it has sunken. Serve.