Melt the dark chocolate in a heatproof bowl over a pot of simmering water, stirring until smooth. Remove from heat and let it cool slightly.
In a separate bowl, whisk the egg whites with a pinch of salt until soft peaks form. Gradually add the maple syrup (or honey) while whisking until glossy and stiff peaks form.
In another bowl, beat the heavy cream until it holds soft peaks. Gently fold in the cocoa powder.
In the bowl with slightly cooled chocolate, add the egg yolks and mix until combined.
Carefully fold in the whipped cream mixture into the chocolate until no streaks remain.
Finally, gently fold in the whipped egg whites in two additions until just combined.
Spoon the mousse into serving glasses or bowls and chill in the refrigerator for at least 15 minutes before serving.
Notes
Top each mousse with fresh berries and a sprig of mint for a pop of color and extra freshness.