01 - Melt chocolate and butter together in a heatproof bowl set over simmering water (double boiler). Stir until smooth; set aside to cool slightly.
02 - In a medium bowl, beat egg yolks with half the sugar and vanilla until pale and creamy. Gradually whisk in the melted chocolate mixture until smooth.
03 - In a separate bowl, whip the heavy cream until soft peaks form; set aside.
04 - In another clean bowl, 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.