01 - Melt the dark chocolate in a heatproof bowl set over a pot of simmering water, stirring constantly until completely smooth. Remove from heat and allow to cool for 5 minutes until just warm to the touch.
02 - In a clean mixing bowl, whisk together the egg yolks with half of the granulated sugar and vanilla extract. Continue whisking for 3-4 minutes until the mixture becomes pale yellow and thickened.
03 - Gently fold one-third of the melted chocolate into the egg yolk mixture to temper it. Once incorporated, fold in the remaining chocolate until fully combined and smooth.
04 - Using an electric mixer or whisk, beat the cold heavy cream in a chilled bowl until soft peaks form. The cream should hold its shape but still have a slight curl when the whisk is lifted. Set aside.
05 - In a separate clean bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually sprinkle in the remaining sugar while continuing to beat until stiff, glossy peaks form.
06 - Fold one-third of the whipped cream into the chocolate base to lighten it. Gently fold in the remaining cream, being careful not to overmix. Repeat the process with the beaten egg whites, adding them in two portions and folding with a spatula until just incorporated.
07 - Divide the mousse evenly among 4 serving glasses or bowls. Cover each with plastic wrap and refrigerate for at least 2 hours or until completely set and firm.
08 - Just before serving, gently toss the strawberries, blueberries, and raspberries with powdered sugar if using. Allow to sit for 2 minutes to draw out some natural juices.
09 - Remove the mousse from the refrigerator. Top each serving generously with the mixed berries and garnish with fresh mint leaves if desired. Serve immediately.