In a small bowl, mix together gelatin and 1/4 cup water. Let stand for 5 minutes while gelatin blooms.
In a medium saucepan, combine frozen raspberries, sugar, and 1/4 cup water. Warm the mixture until the sugar has dissolved and the raspberries are not frozen anymore.
Remove the mixture from the heat source and place into a blender to purée.
Place a sieve over a medium heat proof bowl. Strain the raspberry mixture through the sieve to remove the seeds. Using the back of a spoon, push the raspberry juice through the sieve as much as you can. It's quite time consuming, and you will throw out a lot of the raspberry mixture.
Place the raspberry mixture into the saucepan and heat on low-medium until boiling. Add gelatin and cook for 1 minute.
Cool the raspberry mixture to room temperature.
Pour whipping cream into a metal bowl for your stand mixer. Fit your stand mixer with the whisk attachment.
Whip the cream until stiff peaks form (5-7 minutes).
Fold the raspberry mixture into the whipping cream. Place raspberry mousse into serving containers.
Chill the raspberry mousse for 4 hours before serving.