Prepare the chocolate: In a heatproof bowl, melt the chopped semi-sweet chocolate over a pot of simmering water (double boiler method) or microwave it in 20-second intervals, stirring between each interval, until fully melted and smooth. Set aside to cool slightly.

1. Make the mousse base: In a separate bowl, whip the heavy cream and sugar together with an electric mixer until soft peaks form. Be careful not to overwhip the cream.
2. Combine chocolate and cream: Once the chocolate has cooled slightly, gently fold the melted chocolate into the whipped cream using a spatula. Be careful not to deflate the mixture. Add the vanilla extract and mix until smooth.
3. Add raspberry puree: Carefully fold in the raspberry puree, making sure it’s evenly distributed but not overmixed. The mousse should remain light and airy.
4. Chill the mousse: Spoon the mousse into serving glasses or bowls. Cover and refrigerate for at least 2 hours or until the mousse has set.
5. Prepare the raspberry topping: While the mousse is chilling, toss the fresh raspberries with sugar (if desired) to bring out their natural sweetness. Let them sit for a few minutes to release their juices.
6. Serve: Once the mousse is set, top with fresh raspberries and a drizzle of raspberry syrup (optional). Garnish with chocolate shavings or a dollop of whipped cream if desired.