Put the sugar-free raspberry gelatin mix in a mixing bowl. Pour in the boiling water and whisk for 1 to 2 minutes, until every crystal has dissolved.

Let the dissolved gelatin cool to room temperature. It should remain fluid but no longer feel hot, since hot gelatin can soften the cream cheese mixture and deflate the whipped cream.

Beat the softened cream cheese until smooth. Add the powdered keto sweetener and vanilla extract if using, then mix until evenly incorporated.

Slowly beat the cooled gelatin into the cream cheese mixture. Continue until the color and texture are even, scraping down the sides of the bowl as needed.

In a separate chilled bowl, whip the cold heavy cream to medium-stiff peaks. Stop when the cream holds its shape but still looks smooth and creamy.

Gently fold the whipped cream into the raspberry mixture in portions. Fold in the fresh raspberries if desired, using a light touch so the mousse stays airy.

Spoon the mousse into six small glasses or ramekins. Refrigerate for at least 2 hours, preferably 3 to 4 hours, until firm, then garnish with whipped cream and a few raspberries.