In a heatproof bowl, combine the chopped dark chocolate and cubed butter. Use a double boiler method by placing the bowl over simmering water, or melt in the microwave in 20-second intervals, stirring frequently. Once melted and smooth, remove from heat and let it cool slightly.
In a medium bowl, whisk together the egg yolks and granulated sugar until the mixture becomes pale and creamy. Gradually add the cooled chocolate mixture and vanilla extract, mixing until thoroughly combined and smooth.
In a clean, dry bowl, beat the egg whites with a pinch of salt using a handheld mixer or whisk until stiff peaks form.
Gently fold one-third of the whipped egg whites into the chocolate mixture to lighten it. Carefully incorporate the remaining egg whites, folding just until no white streaks are visible.
In a separate bowl, whip the heavy cream until soft peaks form. Carefully fold the whipped cream into the chocolate mixture until fully combined.
Spoon the mousse into individual serving cups or ramekins. Lightly tap the cups on the counter to eliminate any air bubbles.
Cover the cups with plastic wrap and chill in the refrigerator for at least 2 hours or until set. Just before serving, sprinkle the tops with crushed candy canes and place a fresh mint leaf for garnish.