Prepare a 8� x 8� baking pan with parchment paper and preheat the oven 180�C.
Pour butter, milk, castor sugar and brown sugar in a saucepan, stir it together using medium heat until it just starts to boil.
Remove from heat, then stir in dark chocolate coins until everything is melted and smooth.
Let the chocolate cool for about 5 minutes, then whisk in the eggs and vanilla essence.
Mix in flour (A) and baking soda until they are combined.
In another bowl, toss chopped peanut butter cups with flour (B).
Then, fold in the peanut butter cup into the brownie batter.
Pour the brownie batter into a baking pan, then scoop some peanut butter on top, and swirl around using knife or skewers.
Bake at 180�C for 45 minutes or until the cake is done.
Let it cool slightly before slicing, and ready to serve!