Press undrained raspberries through a sieve; discard seeds. (You should have about 1 cup puree.) Combine the puree and gelatin in a small saucepan; let stand 5 minutes. Cook and stir over medium-low heat until gelatin is dissolved; set aside.
Meanwhile, combine the chocolate pieces, whipping cream, and vanilla in a heavy medium saucepan. Cook and stir over low heat until chocolate pieces are melted. Whisk smooth. Stir in raspberry mixture; heat and stir until smooth.
Transfer to a large mixing bowl. Cover and refrigerate about 3 hours or until thoroughly chilled, stirring occasionally. Beat with an electric mixer just until fluffy. Makes about 3-1/2 cups (enough to fill and frost a four-layer cake).