Pour a teaspoon of vegetable oil into a small rectangular glass dish. Using a paper towel, grease and wipe it clean.
Prepare 2 packets jelly together according to package instructions and reduce 100ml water for a firmer jelly.
Pour into your prepared dish and refrigerate until firmly set. When firmly set, cut into square pieces.
Grease a 23cm bundt pan with vegetable oil and wipe it clean with a paper towel. Evenly spread the jelly pieces on the base of the bundt pan.
Dissolve 1 packet jelly with 250ml/1 cup boiling water. Leave to cool for 15 minutes.
Pour the whipping cream into a blender and add the jelly. Blend for 1 minute and pour over jelly pieces.
Smooth the surface, then lightly bang the bundt pan 2 times on your counter surface (this will ensure the mousse has gone down in-between the jelly). Refrigerate for at least 4 hours, or until firmly set.
To unmold your mousse cake; place a serving plate over the top of the bundt pan and flip it over. Let the mold fall gently onto the plate. This will take 2-3 minutes. If it doesn't come out, fill a sink basin with warm tap water. Dip the outside of the mold into the water for a 2 seconds. Remove from water, dry the outside of the mold, and flip onto serving plate.