For the chocolate mousse:

In a microwave safe bowl, put the chocolate broken into small pieces with the butter.

Do a bain marie (boil some water in sauce pan that is a little smaller than your bowl. Turn off the stove. You can place the bowl with the chocolate on top of the sauce pan or in a bigger bowl that can contain the bowl with the chocolate. Safety first.) The vapor from the water will melt the butter and the chocolate. Once the chocolate starts to melt, start whisking slowly until everything is melted. 

Separate the yolk from the egg whites, but putting the yolk in your mixture on a the time. Whisk the chocolate every time you put a yolk so the heat doesn’t cook the egg. Put it aside.

The egg whites can go in a stand mixer or electric hand mixer. Whip the egg whites on high speed.

Incorporate the sugar to the egg whites in 3 stages. 

The egg whites are ready when they are firm and form stiff peak on  the beaters when lifted. 

With a large spatula, take a third of the egg whites and incorporate vigorously in the chocolate mixture.

Take the second third and incorporate delicately by folding the egg whites into the chocolate to make it airy.

To the same thing with the last third.

Put your chocolate mousse into dessert glasses and in the fridge for at least 4 hours.

For the coffee mousse:

Using a stand mixer or electric hand mixer,  mix the heavy cream, vanilla, and icing sugar on medium high speed until the cream is well whipped. 

It should be firm not runny.

Put the coffee mousse on top of the chocolate mousse in the fridge for 4 hours.

Note: You can add the coffee mousse on top of the chocolate mousse after 3 hours. Just make sure that the chocolate mousse is set before adding anything on top.