Separate the Eggs Carefully: Begin by separating the egg yolks from the whites while the eggs are still cold for easier handling. Place the whites in a large mixing bowl and the yolks in a smaller one. Allow the egg whites to come closer to room temperature as you prepare the other ingredients.
Whisk the Egg Yolks Until Smooth: Using a whisk or fork, beat the egg yolks gently until their color is uniform and the texture is slightly creamy. Set aside for folding later.
Melt Chocolate and Butter Gently: Chop the dark cooking chocolate into small pieces and combine it with the butter in a microwave-safe bowl. Heat in short 30-second intervals, stirring between each, until completely melted and smooth. Remove from microwave and let it cool slightly but keep it warm to maintain fluidity.
Whip the Cream to Soft Peaks: Pour the full-fat cream into a chilled bowl and whip it with a hand mixer or whisk until it forms soft but firm peaks. Be careful not to overwhip, as the cream should be light and fluffy.
Beat Egg Whites with Sugar: Add the caster sugar to the egg whites. Beat the mixture on medium-high speed until it reaches firm peaks — when the peaks hold their shape but still have a slight bend at the tip. This step is crucial for creating the mousse’s airy texture.
Fold Yolks Into Whipped Cream: Gently fold the beaten egg yolks into the whipped cream using a rubber spatula. Use slow, deliberate folding motions to maintain the light texture. Stop once the mixture is just combined, even if there are some streaks remaining.
Incorporate Melted Chocolate: Make sure the melted chocolate is warm but still pourable (around 40°C / 104°F). Slowly pour it into the cream and yolk mixture and fold carefully until the chocolate is evenly distributed, again avoiding overmixing.
Fold in One-Quarter of Egg Whites: Add about a quarter of the whipped egg whites to the chocolate mixture. Fold gently and thoroughly to lighten the mixture before adding the rest.
Combine Remaining Egg Whites: Pour the remaining whipped egg whites into the chocolate mixture. Fold delicately until the mousse is uniform in color and texture, with no visible white streaks remaining. Avoid overfolding to preserve volume.
Chill Until Set: Divide the mousse evenly into four serving glasses or small bowls. Cover and refrigerate for at least 6 hours, preferably overnight, to allow the mousse to fully set and develop its signature airy texture.
Garnish and Serve: Before serving, decorate each mousse with a dollop of whipped cream and sprinkle with chocolate shavings. Fresh raspberries and a mint sprig can add a lovely touch of color and freshness.