01 - Place dark chocolate in a heatproof bowl set over a pan of gently simmering water, creating a double boiler. Stir continuously until completely smooth and melted. Remove from heat source and allow to cool for a few minutes until lukewarm.
02 - In a clean, dry mixing bowl, beat egg whites with a pinch of salt using an electric mixer or whisk until soft peaks begin to form. Gradually sprinkle in half of the granulated sugar while continuing to beat. Continue until stiff, glossy peaks form and the meringue holds its shape.
03 - In a separate bowl, whisk together the egg yolks with remaining granulated sugar and vanilla extract. Whip vigorously until the mixture becomes noticeably pale, thick, and creamy in texture.
04 - Pour the cooled melted chocolate into the egg yolk mixture. Using a rubber spatula, fold gently but thoroughly until completely incorporated and no streaks remain. The mixture should be smooth and uniform.
05 - In a chilled bowl, whip the cold heavy cream with an electric mixer or whisk until soft peaks form. The cream should hold its shape but still have a slight droop at the peak.
06 - Add about one-third of the whipped cream to the chocolate mixture. Fold gently using a spatula, being careful not to deflate the mixture. Repeat with remaining cream in two more additions until fully incorporated.
07 - Gently fold the beaten egg whites into the chocolate mixture in three separate additions. Use a light folding motion to preserve as much air as possible while ensuring even distribution. Stop as soon as no white streaks remain visible.
08 - Spoon or pipe the mousse into individual serving glasses or bowls, filling them about three-quarters full. Cover tightly with plastic wrap and refrigerate for at least 2 hours, or until the mousse is completely set and firm to the touch.
09 - Just before serving, place the prepared strawberries in a bowl. Sprinkle with powdered sugar and drizzle with lemon juice if desired. Toss gently to coat evenly. Let stand for 5 minutes to allow the strawberries to release their natural juices.
10 - Remove the chilled mousse from the refrigerator. Top each serving generously with the prepared strawberries. Finish with chocolate shavings and fresh mint leaves if using. Serve immediately while the mousse is cold and the strawberries are fresh.