01 - Set a heatproof bowl over a pan of simmering water to create a double boiler. Add the chopped chocolate and cubed butter, stirring constantly until completely melted and smooth. Remove from heat and allow to cool for 5 minutes.
02 - In a large mixing bowl, whisk together the egg yolks with half of the sugar (2 tbsp) and vanilla extract. Beat vigorously until the mixture becomes pale yellow and slightly thickened, approximately 3 minutes. Gradually pour in the cooled chocolate mixture while whisking continuously until fully incorporated.
03 - Using a completely clean and grease-free bowl, beat the egg whites with a pinch of salt starting at low speed. Increase to high speed and continue beating until soft peaks form. Gradually add the remaining sugar and beat until stiff, glossy peaks form that hold their shape when the beaters are lifted.
04 - In a separate chilled bowl, whip the cold heavy cream using an electric mixer or whisk until soft peaks form. Be careful not to over-whip or the cream will become grainy.
05 - Gently fold one-third of the whipped cream into the chocolate mixture to lighten it. Add the remaining whipped cream and fold using a rubber spatula with a gentle over-and-under motion until just combined. Some white streaks should remain visible.
06 - Add the beaten egg whites to the chocolate mixture in three separate additions. Fold gently after each addition using a spatula, being careful not to deflate the mixture. Stop folding when the mixture is smooth and airy with no visible white streaks.
07 - Spoon or pipe the mousse into individual serving glasses or ramekins, filling each about three-quarters full. Tap the glasses gently on the counter to release any air bubbles. Refrigerate for at least 2 hours or until the mousse is completely set and holds its shape.
08 - Just before serving, top each mousse with fresh raspberries or strawberries, shaved chocolate, and small mint leaves if desired. Serve chilled with crisp butter cookies or ladyfingers on the side.