In a medium saucepan, combine fresh raspberries and granulated sugar over low heat. Stir for about 5 minutes until the raspberries soften and release their juices. Let it cool slightly before blending until smooth. Strain the puree through a fine mesh sieve to remove seeds.

If using gelatin, sprinkle it over 2 tablespoons of cold water. Allow it to sit for about 5 minutes. Gently heat the mixture until fully melted, then stir into the cooled raspberry puree.

In a large mixing bowl, whip the heavy whipping cream with powdered sugar and vanilla extract using an electric mixer until stiff peaks form.

Fold a third of the whipped cream into the raspberry puree. Gradually add remaining whipped cream in two more additions, folding carefully to maintain airiness.

Spoon the mousse into serving dishes and cover loosely with plastic wrap. Chill for at least 2 hours.

Garnish with fresh raspberries before serving.