1. Make mousse: In a bowl, mix gelatin into 3 Tbsp. cold water. Let stand to dissolve.
2. Combine raspberries and sugar in a medium pot; cook over medium-low heat, stirring, until sugar dissolves and mixture is warm, 5 minutes. Stir in gelatin until blended. Pour into a bowl and set aside to cool.
3. Combine cream and vanilla in a large bowl; whip until cream holds soft peaks. Gently fold in raspberry mixture. Spoon mousse into 6 dessert goblets. Lightly cover with plastic wrap; refrigerate until set, at least 2 hours or up to 2 days.
4. Make sauce: Combine chopped chocolate and 3 Tbsp. water in a microwave-safe bowl; microwave on high until chocolate is melted, 1 minute. Whisk until smooth. Cool slightly and drizzle over mousse before serving.