01 - Melt chocolate and butter together in a heatproof bowl set over simmering water, stirring until smooth. Set aside to cool slightly.
02 - Beat egg yolks with half the sugar and vanilla until pale and creamy. Gradually whisk in the melted chocolate mixture until smooth.
03 - Whip the heavy cream in a separate bowl until soft peaks form. Set aside.
04 - Beat egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar and beat until glossy, stiff peaks form.
05 - Gently fold the whipped cream into the chocolate mixture, then carefully fold in the egg whites in two additions until just combined.
06 - Spoon mousse into four serving glasses. Cover and chill for at least 2 hours, or until set.
07 - Before serving, toss berries with powdered sugar and lemon juice. Spoon over the mousse. Garnish with mint leaves and chocolate shavings if desired.