1. Add the raspberries, granulated sugar, and lemon juice to a saucepan over medium heat. Cook for 4 to 5 minutes, stirring and gently mashing, until the berries break down and become saucy.

2. Press the raspberry mixture through a fine-mesh sieve into a bowl to remove the seeds, then discard the solids.

3. In a small bowl, sprinkle the gelatin over the cold water and let it bloom for 5 minutes.

4. Stir the bloomed gelatin into the warm strained raspberry puree until fully dissolved. Let the mixture cool to room temperature.

5. In a chilled mixing bowl, whip the heavy cream with the powdered sugar and vanilla extract until soft peaks form.

6. Fold the cooled raspberry mixture into the whipped cream in 2 to 3 additions, mixing gently until smooth and airy.

7. Spoon or pipe the mousse into heart-shaped silicone molds and smooth the tops.

8. Refrigerate for at least 4 hours, or until fully set.

9. Carefully unmold and serve chilled with fresh raspberries if desired.