For the cookies:

1. Beat butter and sugar in a large bowl until very well mixed.
2. Add vanilla and melted chocolate in 2 parts. Mix well.
3. Add egg and mix until well incorporated.
4. Sift over this mixture (or do it in a different bowl and then add) the flour, cocoa, salt and baking soda. Do it in 2 parts for easier mixing.
5. Make sure it's very well mixed, leaving no dry spots.
6. Have a large piece of plastic wrap on the counter.
7. Put half of the batter in the form of a shaggy log lengthwise, and roll the plastic (as you would cinnamon rolls) making a cylinder with the cookie dough.
8. It will feel bloated so lightly pierce the plastic wrap in a few places to release air bubbles. The tighter the better. The ends of the plastic will be rolled; tuck them underneath.
9. Repite with the rest of the dough.
10. Refrigerate these logs until firm enough to cut, about 1 hour and upto 1 week (or frozen 1 month). Place them in a smooth place in the fridge so that they keep their round shape as much as they can.
11. Preheat oven to 325ºF/170ºC.
12. Line baking sheets with parchment paper,
13. Cut the logs into rounds about 1/4 inch thick.
14. Re-shape them again with your hand to create the best circular shape you can and arrange on the sheets an inch or two apart.
15. Bake for 15 to 20 minutes, until they are somewhat firm to the touch. See Notes, below. 
16. Let cool completely on wire rack before filling. Let the filling harden or set before eating.

For the filling:

1. Mix all ingredients in a large bowl until creamy and smooth.
2. Add more sugar if you want a firmer filling. Powdered sugar and butter brands are different so you might need to adjust the amounts.
3. Refrigerate until ready to use.