Pass the berries through a food mill fitted with a fine disk or force them through a fine sieve, mashing with a wooden spoon, into a medium bowl. You'll have about 1/2 cup puree; set it aside and discard the contents of the strainer. Put the chopped chocolate in a medium bowl. In a small saucepan, heat the cream until just boiling. Pour the hot cream over the chopped chocolate; whisk to blend. Stir in the raspberry puree and the salt. Refrigerate the mixture until completely chilled, about 1 hour.

Pour the cocoa powder onto a plate. With a melon baller or spoon, scoop the chocolate and shape it into 1-inch balls. If the truffles are very soft, put them on a baking sheet and refrigerate briefly to firm. Roll the shaped truffles in cocoa, coating them thoroughly. If sealed and refrigerated, the truffles will keep for about a week.