For the Brownies:

1. Preheat oven to 325°F and butter an 8 x 8-inch baking pan. I also lined mine with a sheet of parchment paper overhanging the sides – this is totally optional but makes getting the brownies out of the pan much easier.
2. Place the chocolates and butter in a large microwave save bowl. Microwave in 20-second increments, stirring after each session, until the chocolate is mostly melted. Stir to completely melt the chocolate.
3. Whisk in the sugar, instant espresso, salt, and peanut butter until smooth.
4. Whisk in the eggs, one at a time, and the vanilla until smooth.
5. Gently mix in the flour and stir until it is just combined and the batter is smooth.
6. Pour roughly half the batter into the prepared baking pan. Top with a layer of the peanut butter cups. Top the peanut butter cups with the remaining batter, smoothing out the top as much as possible.
7. Bake for 25-30 minutes or until the brownies are cooked through. Allow the brownies to cool completely in the pan before frosting.

For the Frosting:

1. In the bowl of a stand mixer, beat the butter and peanut butter together until creamy and fluffy, about 1 minute. Add in a tablespoon of milk and whisk to combine.
2. Add in the salt and powdered sugar and mix on low to incorporate the sugar. Increase the speed and beat for another couple minutes or until the frosting is fluffy and smooth. If the frosting is too thick, beat in another tablespoon of milk until desired consistency is achieved.
3. Frost the brownies generously with the frosting. If desired, garnish with additional chopped peanut butter cups or chocolate chips.