01 - Set a heatproof bowl over a pan of simmering water to create a double boiler. Melt the chocolate with butter, stirring constantly until smooth and glossy. Remove from heat and allow to cool for 5 minutes.
02 - In a large mixing bowl, whisk egg yolks with half the sugar (2 tablespoons) and vanilla extract until the mixture becomes pale and creamy, about 3 minutes. Gradually pour in the cooled chocolate while whisking continuously until fully incorporated.
03 - Using a clean, grease-free bowl, beat egg whites with salt until soft peaks form. Slowly add remaining sugar and continue whipping until stiff, glossy peaks develop, approximately 4-5 minutes.
04 - In a separate chilled bowl, whip cold heavy cream until soft peaks form, being careful not to overbeat.
05 - Gently fold the whipped cream into the chocolate mixture using a rubber spatula. Make light, circular motions until just combined—some streaks are acceptable.
06 - Fold in the beaten egg whites in three separate additions. Use a lifting motion to maintain airiness until the mixture appears smooth and uniform. Avoid overmixing to preserve the airy texture.
07 - Divide mousse evenly among individual serving glasses or ramekins. Refrigerate for a minimum of 2 hours until completely set and firm.
08 - Top each portion with fresh berries, shaved chocolate, and mint leaves before serving.