Madeleine's chocolate mousse

Method

• Step 1
  Place dark chocolate and butter in a heavy-based saucepan over low heat. Cook, stirring occasionally, for 3 to 4 minutes or until chocolate is just melted and mixture is smooth (do not overheat). Remove from heat. Set aside to cool until lukewarm (see notes).
• Step 2
  Using an electric mixer on low speed, beat egg yolks, 1 at a time, into chocolate mixture until just combined.
• Step 3
  Wash and dry beaters. In a separate bowl, beat egg whites until stiff peaks form. Gradually beat in sugar, 1 tablespoon at a time, beating until sugar is dissolved after each addition. Add a little of the egg white mixture to the chocolate mixture, folding to combine. Add the chocolate mixture to remaining egg white mixture. Fold gently until just combined.
• Step 4
  Spoon chocolate mixture among 8 x 1/2-cup-capacity serving dishes. Refrigerate for 3 hours or overnight (see notes).
• Step 5
  Remove mousse from fridge 30 minutes before serving. Dollop with whipped cream and dust with cocoa powder. Serve.