For the Cookie Cups
Preheat oven to 350°. Grease a mini muffin tin very well and set aside. (You will make 3 batches)
Cream melted butter and brown sugar together until smooth. Add in egg and vanilla and mix until smooth. Scoop in peanut butter and stir until incorporated completely. Pour in flour and baking soda and mix until just combined.
Scoop about 1 1/2 tablespoons of dough (enough to fill the cookie tins 3/4 of the way) for each cookie into the muffin tin wells. Repeat this process for the remaining dough. You will need to make 3 batches so fill as many pans as you have until the second batch (2 mini muffin tins should fit at the same time.) Place pans in preheated oven and bake for 10-12 minutes or until edges are set. Edges will be set but middle will still be soft and underdone. This is what makes them chewy.
IMMEDIATELY when they come out of the oven, make a deep well into each of the cookie cups with anything you have with a rounded head. I like to use the arm of my rolling pin because its the perfect size and rounded or I have also used a melon baller. Let them cool in their pans for 10 minutes before transferring to a wire rack or they will be too soft to handle. While cookies are cooling, prepare your simple chocolate ganache.
For the Chocolate Ganache
Heat the heavy cream in a small saucepan until warm to the touch. You can also do this carefully in the microwave for about 30 seconds. Add in chocolate chips and mix until the warm heavy cream melts the chips completely. Mixture should be very smooth and satiny, it should not be thick. If it is too thick it may not stay creamy after the first day so add more cream. Spoon the ganache into the well of the cooled cookies right to the top. Once you filled all of your cookies, sprinkle your chopped reese's on top.
Cookies will stay soft for up 10 days at room temperature in an airtight container.