Place chocolate and butter in medium heatproof bowl. Place bowl over a medium pan of simmering water (don’t let the water touch the bottom of the bowl). Stir, using a metal spoon, until melted and smooth. Remove bowl from heat. Whisk in yolks.
Using an electric mixer, whisk whites until soft peaks form. Transfer whites to a medium bowl. Using same bowl (I don’t clean it!), whisk cream until soft peaks form. Whisk half the cream into chocolate mixture. Then fold chocolate mixture into remaining cream. Add whites and fold gently to combine.
Divide mousse among serving dishes. Cover and refrigerate for at least 3 hours (or overnight).
Notes
I use a 45% cocoa chocolate, if you like it extra dark, rich and a little bitter use a higher percentage cocoa chocolateFor entertaining (or for the kids) I serve my mousse in little espresso cups. It makes about 12-14 espresso cups. If you want a sweeter mousse, use milk chocolate. This will save having to add sugar to the mousse mixture.