Preheat the oven to 400°.
Unwrap the thawed puff pastry and cut 3 x 3 inch squares. Flatten the puff pastry with your hands or with a rolling pin. You'll want 24 of these squares.
Spray the mini cupcake tins with a non stick spray (this is very important) and don't wipe the spray off of the flat top of the pans.
Place each square of the puff pastry in a mini cupcake slot and tuck into the slot. It's okay for the edges of the pastry to stick out.
If you have two tins, place the first one in the fridge while you prep the second one. If you have one tin, place in the fridge for 10 minutes after it's prepped.
After they chill, bake the pastry cups for 15 minutes at 400°.
Remove them from the oven and let them cool in the pan for a few minutes.
Take a tablespoon measurer and squish down the center of the pastry. Be gentle, you don't want to break or crush the pastry.
Fill each cup with about 2 tespoons of the chocolate ganace.
If you want an overflowing, gooey s'mores cup then place one whole marshmallow standing on top of the ganache. You can cut the marshmallow in half if you want. (or use a spoonful of minimarshmallows)
Bake in the 400° oven for about 3 minutes. The marshmallow will rise a lot and then fall as they cool.
Let cool for at least 5 minutes before you try to remove them from the tins.
Take a butter knife of small offset spatula to help you scoop the cups out of tin. Be gentle!
Storage: place in an airtight container and enjoy within a day or two of baking. Refrigerate if you want to store longer and toast before you eat.
If you happen to have any ganache left over you can store it in the freezer until next use!