Slit the vanilla pod and scrape out the vanilla seeds with the knife.
Take a pot.
Put the frozen raspberries, sugar, vanilla seeds, and the scraped vanilla pod in the pot.
Bring it to a boil on the stove for a few minutes.
Then remove the pot from the heat.
Fish the gelatin out of the water. Squeeze the water out of the gelatin.
Stir the gelatin into the pot with the hot mixture.
Let the gelatin melt.
Let it all cool down a bit.
Take a new, large bowl. Pour heavy cream into the bowl.
Whip the cream to a light foam. Whip it a bit lighter than if you were going to use it for hot cocoa.
Fold a bit of the whipped cream into the pot with the raspberry mixture.
Then gently pour the raspberry mixture into the bowl with the whipped cream. Gently fold it together in the bowl.
Place the bowl in the fridge overnight.
Stir the raspberry mousse a bit with a spoon.
Put the raspberry mousse in a piping bag with a star tip.
Decorate your cake or dessert with the raspberry mousse.
Keep the display on
Notes
You can also choose to spread the raspberry mousse over your cake or in the layers of a layer cake. In this case, you don't need to refrigerate the mousse overnight - instead, you can just use the mousse immediately.