Break the chocolate into a stewpan, add sugar, water and cognac to it. Put on a slow fire while stirring constantly until the chocolate has completely dissolved.
Take two plates in one, put ice and some water. Place the second on top and pour the chocolate into it. Beat with a mixer until the mousse begins to thicken, it takes about 5-10 minutes.
Chocolate mousse is ready, transfer to a bowl and serve with whipped cream or grated chocolate.