1. Line baking sheets with waxed paper. Heat cognac in saucepan just to simmer. Remove from heat. Add cream. Bring to simmer.
2. Process chocolates and powdered sugar in food processor 1 to 2 minutes, until finely chopped. With machine running, add cream mixture in steady stream. Process until smooth. Scrape into bowl. Refrigerate just until thick enough to hold shape, about 1 hour.
3. Drop by rounded teaspoons onto baking sheets. With hands, quickly shape into balls. (Refrigerate to firm while working if necessary.) Place in freezer 30 minutes.Coating:
4. Sift cocoa and sugar into bowl. Add truffles, three at a time; toss to coat. Refrigerate in airtight containers up to 1 month. Serve at room temperature.