Melt the chocolate and butter together using a double boiler or microwave, stirring until smooth. Let cool slightly.In a bowl, whisk the egg yolks with the sugar until pale, then stir into the melted chocolate along with the vanilla.Whip ½ cup of the heavy cream until soft peaks form and fold it into the chocolate mixture.In another bowl, whip the remaining 1 cup of heavy cream until medium peaks form.Gently fold the whipped cream into the chocolate mixture in batches until fully combined.Spoon the mousse into six serving glasses.Cover and refrigerate for at least 4 hours, or overnight for the best texture.Garnish with whipped cream, chocolate shavings, or fresh berries before serving.
