Coarsely chop the chocolate. Combine in a bowl with the butter and melt over a double boiler. Set aside and leave to cool.
Whisk the eggs and sugar until foamy. Combine the flour, cocoa powder, cream of tartar powder, cinnamon and salt in another bowl. Stir the cooled, liquid chocolate mass into the egg mass, then fold in the dry ingredients and scraped out vanilla seeds. Cover the bowl and chill for 2 hours.
Make sure to