1. Prepare the Raspberry Puree:

1. In a small saucepan over medium heat, combine the fresh raspberries and granulated sugar.
2. Cook, stirring occasionally, until the raspberries soften and release their juices (about 5-7 minutes). The raspberries should break down easily.
3. Pour the mixture through a fine-mesh strainer set over a bowl, pressing on the solids with a spoon or spatula to extract as much puree as possible. Discard the seeds.
4. Stir in the lemon juice (if using) into the strained raspberry puree.
5. Set the puree aside to cool completely to room temperature. This is important, as warm puree can deflate the whipped cream.

2. Prepare the Gelatin (Optional, for Firmer Mousse):

1. If using gelatin, sprinkle the gelatin powder evenly over the cold water in a small bowl.
2. Let it sit for 5 minutes to bloom (soften).
3. Microwave the bloomed gelatin for 5-10 seconds, or until it’s completely melted and clear. Do not overheat.
4. Let the melted gelatin cool slightly (but not set). It should be liquid but not hot.
5. Once the raspberry puree has cooled to room temperature, stir in the melted gelatin until evenly combined.

3. Whip the Cream:

1. In a large, chilled mixing bowl, combine the chilled heavy whipping cream, powdered sugar, and vanilla extract.
2. Using an electric mixer (handheld or stand mixer), beat the cream on medium-high speed until stiff peaks form. Be careful not to over-whip, or the cream will become grainy. Stiff peaks should hold their shape when the beaters are lifted.

4. Fold in the Raspberry Puree:

1. Gently fold the cooled raspberry puree into the whipped cream, one-third at a time.
2. Use a rubber spatula and a light, folding motion to combine the ingredients. Do not stir vigorously, as this can deflate the whipped cream.
3. Continue folding until the puree is evenly incorporated and the mousse is a uniform light pink color.

5. Chill the Mousse:

1. Spoon or pipe the mousse into individual serving glasses or dessert bowls. A piping bag fitted with a large tip creates a more elegant presentation.
2. Cover the glasses or bowls with plastic wrap.
3. Refrigerate for at least 2 hours, or preferably longer (up to overnight), to allow the mousse to firm up and the flavors to meld.

6. Serve:

1. Garnish with fresh raspberries before serving.
2. Serve chilled.