For the cake:

1. Mix chocolate and cocoa powder in a medium bowl. 
2. Add boiling water all at once, leave for 30 seconds and then stir until the chocolate is completely dissolved. It might seem at first that it won’t but it will. That’s why we add almost boiling coffee or water, to soften the chocolate.
3. Preheat the oven to 350°F/180°C.
4. Grease or spray an 8 or 9-inch (22 or 24cm) round cake pan with removable bottom. Sprinkle with fine breadcrumbs or flour and refrigerate while making the batter. 
5. Beat oil and sugars for 1 minute in a large bowl. 
6. Add eggs one at a time, and beat well each time. 
7. Add the vanilla and mix.
8. Sift flour, bicarbonate, baking powder, and salt.
9. Add to the oil mixture in 3 parts, alternating with the chocolate mixture and sour cream in 2 parts. That means that you start and end with flour. 
10. Mix very well but do not overbeat the batter at this point.
11. Pour into the prepared (cold) pan. 
12. Bake for about 40 minutes, or until a tester comes out dry. If you use 2 pans it will take between 30-35 minutes. 
13. Let it cool on a wire rack. 
14. Carefully remove from the pan and let it cool completely before filling (even better if you do it the next day). 
15. If not using immediately, wrap well and either refrigerate for a few days or freeze for a month or more. 

For the filling:

1. Make filling as directed in the recipe, above in the Ingredients
2. Cut the cold cake in two and put the bottom layer on a flat surface. 
3. Fill with about half the chocolate mixture and spread, leaving about half an inch without filling near the edges.
4. Top with the remaining cake round and press lightly. The filling will squish to the edges. 
5. Frost sides and top with a metal spatula or knife. Let dry until it develops a harder outer layer.