Prepare the Eggs: Start by separating the eggs while they are cold from the refrigerator. Place the egg whites in a large, clean mixing bowl and the yolks in a smaller bowl. Allow the whites to come closer to room temperature while you prepare the other ingredients, as this helps achieve better volume when whipped.
Melt the Chocolate and Butter: Break the dark cooking chocolate into smaller pieces and combine with the unsalted butter in a microwave-safe bowl. Heat in short intervals of 30 seconds, stirring thoroughly between each, until fully melted and smooth. Set aside to cool slightly but keep it warm and fluid (ideally around 40°C/104°F).
Whip the Cream: Pour the full-fat cream into a chilled bowl and beat it with a hand or stand mixer until it forms stiff peaks. Be cautious not to overbeat, as this can cause the cream to turn grainy or start separating.
Beat the Egg Whites with Sugar: Add the superfine sugar to the egg whites. Using clean beaters, whip the mixture until firm peaks form—when the peaks stand upright but still have a slight curve at the tip. This texture helps maintain the mousse’s airy lightness.
Combine Egg Yolks and Cream: Gently fold the egg yolks into the whipped cream using a rubber spatula. Use slow, deliberate folding motions to keep as much air in the mixture as possible. Some streaks are fine; avoid over-mixing.
Incorporate the Chocolate: Check the melted chocolate’s temperature—it should be warm and pourable. Fold the chocolate gradually into the cream and yolk mixture, again using gentle folding to preserve air and texture.
Add Egg Whites to the Mixture: Start by adding about one-quarter of the whipped egg whites into the chocolate mixture to lighten it. Fold carefully until just incorporated. Then add the remaining whites, folding delicately until the mixture is uniform with no visible white streaks. This final fold should be gentle to avoid deflating the mousse.
Chill the Mousse: Spoon the completed mousse evenly into four serving dishes or glasses. Cover and refrigerate for at least 6 hours, or preferably overnight, to allow the mousse to set properly and develop its signature airy texture.
Garnish and Serve: Before serving, top with a dollop of freshly whipped cream and finish with fine chocolate shavings. Optionally, fresh raspberries and a sprig of mint add a lovely touch of color and freshness.
