01 - Place the dark chocolate in a heatproof bowl set over a pan of simmering water, stirring constantly until completely smooth. Remove from heat and let cool until lukewarm.
02 - In a chilled mixing bowl, beat the cold heavy cream with a hand mixer until soft peaks form. Cover and refrigerate until ready to use.
03 - Using a clean bowl and whisk attachment, beat the egg whites with a pinch of salt until soft peaks appear. Gradually sprinkle in the sugar while continuing to beat until stiff, glossy peaks form.
04 - Whisk the egg yolks and vanilla extract into the cooled melted chocolate until fully incorporated and smooth.
05 - Gently fold the refrigerated whipped cream into the chocolate mixture using a spatula, being careful not to deflate the air pockets.
06 - Add one-third of the beaten egg whites to the chocolate mixture and fold gently to lighten. Repeat with remaining egg whites in two additions until the mousse is smooth and airy.
07 - Divide the mousse evenly among serving glasses or bowls. Refrigerate for at least 2 hours or until completely set and firm.
08 - Combine raspberries, sugar, and lemon juice in a saucepan over medium heat. Cook for 4-5 minutes, stirring frequently, until raspberries break down and the sauce thickens slightly.
09 - Press the raspberry mixture through a fine mesh sieve into a clean bowl, discarding the seeds. Let cool completely at room temperature.
10 - Spoon the cooled raspberry coulis over the chilled chocolate mousse. Garnish with fresh raspberries and mint sprigs if desired. Serve immediately.