Put the chopped chocolate in a large bowl. Heat the crème fraiche with stevia and vanilla and, when it comes to a boil, pour the chocolate while stirring. Allow the mixture to cool for 20 minutes and then cover with plastic wrap. Put the mixture in the refrigerator for at least 4 hours or until the chocolate thickens. When ready, use a spoon to make small chocolate balls. Refrigerate again for about 2 hours, sprinkle with cocoa powder and serve immediately.