Take ¾ cup of water in a pan and sprinkle the agar agar in it. Keep a side (20-30 mins) I used agar agar powder, so it dissolved easily.
Meanwhile dissolve cocoa powder and corn flour in ¼ cup of lukewarm milk. Boil rest of the milk with sugar.
Add the coco-corn flour mixture to the boiling milk with continuous stirring. Boil for 2-3 minutes with constant stirring.
Heat the agar agar dissolved water and bring to boil. It will be thickened and like a transparent jelly like thing.
Add it to the chocolate mixture and mix continuously. Switch off the flame and keep stirring until it's almost cooled down. (Stirring is to avoid formation of a layer while cooling down).
Mix in the cream and vanilla essence. Stir well.
Refrigerate this mixture to get the perfect set chocolate mousse. I did this by placing the mixture in freezer for approx 20 minutes (Don’t let it set for more time)
Or you can also pour the mixture directly to the mousse moulds that you are going to serve and set it as such in the fridge (not in freezer) until serving time.