In a small bowl, sprinkle the unflavored gelatin evenly over ¼ cup of cold water. Let it sit undisturbed for 5–10 minutes to bloom and soften.

In a small saucepan, combine the raspberries, granulated sugar, and ¼ cup of water. Heat over medium heat until the berries soften and begin to break down, stirring occasionally.

Transfer the raspberry mixture to a blender and process until smooth. If a seedless texture is preferred, pour the purée through a fine-mesh strainer back into the saucepan, using a spoon to press through the liquid.

Stir the softened gelatin into the warm raspberry mixture. Return the saucepan to medium heat and bring the mixture to a gentle boil, stirring constantly for 1 minute to fully dissolve the gelatin. Remove from heat and allow to cool to room temperature (approximately 70–80°F).

In a large chilled mixing bowl, beat the cold heavy cream until stiff peaks form. Take care not to over-whip, as this can affect the mousse's texture.

Gently fold the cooled raspberry mixture into the whipped cream using a silicone spatula. Mix until fully incorporated and smooth, being careful not to deflate the cream.

Spoon or pipe the mousse into serving glasses or ramekins. Cover and refrigerate for at least 4 hours, or until set. For best results, chill up to 24 hours before serving.