Makes about 4 cups / 950 ml (six 5-ounce servings)
4½ ounces (130 g) 70% cacao dark chocolate, such as Valrhona Guanaja or Lindt, or other premium chocolate near the same percentage cacao
½ cup plus 1 tablespoon (130 g) heavy whipping cream (36% milk fat)
4 large (200 g) eggs, separated (80 g yolks/120 g whites)
½ teaspoon cream of tartar (optional, but recommended)
2 tablespoons plus 1½ teaspoons (30 g) sugar
Special equipment
1 (24-inch) disposable pastry bag (optional)
Make the ganache. If needed, chop the chocolate into even pieces. Place the chocolate in a small heatproof bowl. In a small saucepan over medium-high heat, heat the cream to scalding (the cream will just begin to boil; do not let it reach a full boil). Pour the hot cream over the chocolate. Shake the bowl gently to settle all the chocolate into the hot liquid, then let it sit for about 10 seconds without stirring. Using a whisk or silicone spatula, begin vigorously stirring in the center, starting with small tight circles and gradually expanding out in larger concentric circles to the edge of the bowl as the mixture blends. Continue stirring until the mixture is perfectly emulsified (smooth and creamy). Using a silicone spatula, scrape down the sides of the bowl and stir any chocolate clinging to the sides into the mixture. Set the bowl aside for the ganache to cool slightly. The ganache should feel slightly warm but not hot (about 104°F/40°C) when the eggs are added.
Make the meringue. In the very clean bowl of a stand mixer fitted with the whisk beater (or in a mixing bowl using a handheld electric mixer), add the egg whites and cream of tartar, if using. Begin beating the egg whites on medium-low speed. When the egg whites are foamy, sprinkle in half the sugar and continue beating. When the egg whites hold soft peaks, sprinkle in the remaining sugar. Continue beating until the egg whites hold stiff peaks when the beater is raised.
Incorporate the egg yolks. Add the egg yolks all at once to the meringue. Beat on medium-low speed just until the yolks are incorporated. If needed, remove the whisk beater and use it by hand to finish incorporating the yolks to avoid overbeating the meringue while ensuring the yolks are fully incorporated.
Finish the mousse. Check the temperature of the ganache by dipping the tip of your finger into it and touching the ganache to the center spot just below the rim of your bottom lip. The ganache should feel close to body temperature, or around 104°F/40°C). If it feels cold, gently warm it to body temperature using a water bath or in 5-second increments in the microwave. Stir the chocolate and check its temperature again. If needed, transfer the ganache to a bowl large enough to accommodate the mixture when the meringue is added.
Using a silicone spatula, carefully incorporate about one-third of the beaten eggs into the ganache, moving around the edge of the bowl and turning the bowl in the opposite direction of the spatula as you fold the eggs up, over, and through the ganache. Do not overfold it; a few streaks of the eggs will remain. Scrape the remainder of the beaten eggs into the mixture all at once and carefully incorporate them in the same manner. Stop folding as soon as the mixture looks uniform in color. Minimize the stirring and folding as much as possible to maintain the light and fluffy texture.
You can spoon the mousse into serving glasses, or, if using a pastry bag, place the bag into a very tall glass and fold the edges down over the edge of the glass to create a wide opening. Gently scrape the mousse into the bag. Lift up the edges of the bag and shake the bag up and down in three or four quick movements while holding it at the top so that the mousse moves down toward the tip. Tightly twist the top closed to prevent the mousse from escaping (use a clip or wrap any excess around your finger to help keep it closed). Pointing the tip up at a slight angle to prevent the mousse from escaping, cut off the tip of the bag to make an opening about 1¼ inches across. Gently pipe the mousse into serving glasses.
Refrigerate the mousse for at least 5 hours, ideally overnight, to ensure it is fully set. Just before serving, set the mousse out for about 10 minutes at room temperature. Using a microplane grater, shave chocolate over the top. Serve.