01 - Melt the dark chocolate in a heatproof bowl over a pan of gently simmering water, creating a double boiler. Stir constantly until smooth and fully melted. Remove from heat and let cool slightly until lukewarm to the touch.
02 - In a clean, grease-free mixing bowl, beat the egg whites with a pinch of salt using an electric mixer or hand whisk until soft peaks form. Gradually add half of the granulated sugar while continuing to beat until stiff, glossy peaks form.
03 - In a separate bowl, whisk the egg yolks with the remaining granulated sugar and vanilla extract. Whisk vigorously until the mixture becomes pale in color and thickened in texture, approximately 2-3 minutes.
04 - Fold the cooled melted chocolate into the egg yolk mixture using a rubber spatula. Gently incorporate until fully combined and uniform in color, being careful not to overmix.
05 - In a chilled bowl, whip the cold heavy cream using an electric mixer or whisk until soft peaks form. The cream should hold its shape but still have a slightly loose texture.
06 - Gently fold the whipped cream into the chocolate and yolk mixture using a rubber spatula. Add in thirds and use a folding motion to incorporate while maintaining as much air as possible.
07 - Carefully fold the beaten egg whites into the chocolate mixture in three separate additions. Use a light hand to fold from the bottom up, taking care not to deflate the mousse or lose the airy texture.
08 - Spoon or pipe the mousse into individual serving glasses or bowls. Cover each container with plastic wrap and refrigerate for at least 2 hours, or until the mousse is fully set and firm to the touch.
09 - Just before serving, place the halved strawberries in a bowl. Toss with powdered sugar and lemon juice if using, gently coating the berries. Let sit for 5 minutes to allow the flavors to meld and the sugar to dissolve slightly.
10 - Remove the chilled mousse from the refrigerator. Top each portion with the prepared strawberries, arranging them attractively on the surface. Finish with chocolate shavings or curls and fresh mint leaves if desired. Serve immediately.