Line baking sheets with waxed paper. Heat cognac in saucepan just to simmer. Remove from heat. Add cream. Bring to simmer.
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.
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.
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.
