Start out by melting the chocolate chips and water in a pot on low heat. When they are melted, stir in the coffee.
Separate the eggs, the whites go into a mixing bowl and the yolks into a large enough bowl that you can mix them with a fork and not make a mess.
Beat the egg yolks until they are frothy and a paler yellow.
Temper the eggs by taking a large spoonful or two of the hot chocolate mixture and whisking them into the beaten eggs. Remove the chocolate from the stove and add the beaten egg yolks to the melted chocolate in the pot, rapidly mixing to make sure you don't get any lumps in it.
Return the chocolate/egg mixture to the stove and cook for 2-3 minures more, making sure to get the chocolate hot enough to bring the yolks to a safe 165 °F temperature. The mixture will be steamy, NOT boiling, and will thicken up slightly.
Cool the chocolate mixture until lukewarm. I put mine in a glass measuring cup and into the freezer while preparing the other ingredients and stir every couple of minutes, works like a charm.
To make the whipped cream version, beat the whipping cream until thick and soft peaks form. Beat in the sugar and vanilla. Fold into the chocolate mixture gently until combined.
To make the old fashioned version with raw egg whites, beat the egg whites until they are stiff, adding the sugar slowly. Beat the vanilla until combined. Fold into the chocolate mixture gently.
Divide into 6 servings and chill for a few hours before serving.
To be safe I suggest that you make the whipped cream version and not the egg white version. Remember, there are risks to eating raw eggs and raw egg products. This is simply showcasing the old method of making chocolate mousse.
The calories are far less when you do use the egg whites. Perhaps you could switch them to pasteurized egg whites instead and try those instead to cut calories. I personally like the texture with egg whites.