Place the gelatin sheets in a bowl of cold water (do not use hot water or the gelatin will completely dissolve).

Pour the raspberry purée into a saucepan. Once it starts to warm up, add the gelatin and whisk until the gelatin is fully dissolved.

Let the mixture cool while you whip the cream. In a bowl, whip the heavy cream until it thickens and forms stiff peaks. Once you have whipped cream, gently pour in the cooled raspberry purée.

Do not use a whisk to mix; instead, gently fold the mixture with a wooden or silicone spoon so the whipped cream stays fluffy. Now you can use the mousse to fill the bottom of your cake or dessert.