In a small saucepan, heat approximately 1 inch of water to 20-25 °C. Place chocolate into bowl over heated water, making sure the water and bowl do not touch. Cover the bowl with a tea or kitchen towel. Allow to melt, which may take 15-30 minutes.
In another small saucepan, heat the cream until just starting to bubble. Pour over melted chocolate, then add the egg yolks. Stir to combine thoroughly. Set aside.
In a stand mixer fitted with the whisk attachment, add the egg whites. Whisk egg whites until starting to get foamy. Add sugar a tablespoon at a time, very slowly while continuing to whisk at a high speed until peaks start to foam.
Pour chocolate into a large bowl, and carefully add about ⅓ of the egg white mixture to the chocolate. Gently fold 3 to 4 times. Continue to add the remaining merengue to the chocolate, folding very gently.
Divide mousse into 6 serving bowls, and allow to set in the refrigerator for approximately 1 hour.