Line an 8x8 or 9x9 inch baking dish with parchment paper or foil, letting the edges over hang from the pan so you can easily lift the brownies out later. Preheat the oven to 325 degrees F.
Add the butter and 2 ounces milk chocolate to a medium size mixing bowl that is microwave safe. Microwave the butter and chocolate on 30 second intervals, stirring after each interval until melted and smooth. This takes me about 1 minute and 30 seconds. To the melted chocolate mixture add the sugar and instant coffee, whisking until completely combined. Now add the vanilla, and eggs, whisk until smooth. Next, stir in the cocoa powder, flour and salt until smooth and just combined. Try not to over mix the batter, it will be thick.
In another small mixing bowl, mix together the peanut butter, 1 tablespoon butter and powdered sugar until smooth.
Pour half the brownie the batter into the prepared pan. Next, dollop the peanut butter mixture onto the brownie batter and sprinkle with the remaining 2 ounces chopped chocolate. Now layer the remaining batter on top of the other layers. Gently spread the batter to the sides if needed. Don't worry if all the batter does not cover the whole surface area, it will smooth out as the brownies bake.
Bake for 25-30 minutes, until the brownies are set on top. Allow the brownies to cool completely before frosting.
Meanwhile, to make the frosting, heat the cream in a saucepan until just below a boil. Remove from the heat and stir in the chocolate and peanut butter until melted and smooth. Add the vanilla and bourbon until smooth and creamy. Set the frosting aside until the brownies are cool enough to frost.
Once the brownies are cool, spread the frosting over top. Sprinkle with chopped Reese's. Allow the frosting to set about 30 minutes then slice into bars. Brownies can be stored at room temp or in the fridge for about four days.