01 - Place dark chocolate in a heatproof bowl over a pan of simmering water. Stir continuously until completely smooth and melted. Remove from heat and let cool until lukewarm.
02 - In a chilled bowl, beat cold heavy cream with a hand mixer until soft peaks form. Place in refrigerator to maintain structure until ready to fold.
03 - Beat egg whites with a pinch of salt until soft peaks appear. Gradually sprinkle in sugar while beating continuously. Continue until stiff, glossy peaks form and sugar is fully dissolved.
04 - Whisk egg yolks and vanilla extract into the cooled melted chocolate until thoroughly incorporated and smooth.
05 - Gently fold the refrigerated whipped cream into the chocolate mixture using a spatula. Incorporate just until no white streaks remain, being careful not to deflate.
06 - Add beaten egg whites in three portions. Fold each addition gently before adding the next. Continue until mixture is uniformly smooth, airy, and no white streaks are visible.
07 - Distribute mousse evenly among serving glasses or bowls. Refrigerate for a minimum of 2 hours until completely set and firm to the touch.
08 - Combine raspberries, sugar, and lemon juice in a small saucepan over medium heat. Cook for 4 to 5 minutes, stirring frequently, until berries completely break down and sauce lightly thickens.
09 - Remove saucepan from heat. Press mixture through a fine mesh sieve into a clean bowl, discarding seeds. Allow coulis to cool completely at room temperature before refrigerating.
10 - Spoon cooled raspberry coulis over the surface of chilled chocolate mousse. Garnish with fresh whole raspberries and optional mint sprig. Serve immediately.