01 - Place chopped dark chocolate in a heatproof bowl set over a saucepan of simmering water, creating a bain-marie. Stir constantly until completely smooth and melted. Remove from heat and allow to cool for 5 minutes.
02 - Whisk egg yolks with half the sugar (1.5 tablespoons) and vanilla extract in a medium bowl. Beat continuously until the mixture becomes pale in color and creamy in texture, approximately 3 minutes.
03 - Pour the cooled melted chocolate into the egg yolk mixture. Using a rubber spatula, gently fold together until fully incorporated, being careful not to deflate the mixture.
04 - In a clean, dry bowl, beat egg whites with a pinch of salt using an electric mixer or whisk until soft peaks form. Gradually sprinkle in the remaining sugar and continue beating until stiff, glossy peaks form.
05 - In a separate chilled bowl, whip the cold heavy cream until soft peaks form. The cream should hold its shape but still appear slightly loose.
06 - Gently fold the whipped cream into the chocolate base. Add the beaten egg whites in two separate additions, folding lightly after each addition until just combined. Stop when no white streaks remain.
07 - Divide the mousse evenly among 4 serving glasses or ramekins. Refrigerate for at least 4 hours or overnight until completely set and firm.
08 - Combine raspberries, sugar, lemon juice, and water in a small saucepan. Cook over medium heat for 5 to 7 minutes, stirring occasionally, until berries break down completely and the mixture slightly thickens.
09 - Remove the saucepan from heat. Press the raspberry mixture through a fine mesh sieve using a spoon or spatula to remove all seeds. Allow the coulis to cool completely at room temperature.
10 - Spoon or drizzle the cooled raspberry coulis over the chilled chocolate mousse just before serving. Garnish generously with fresh raspberries, chocolate shavings, and mint leaves if desired.