Makes about 2 cups/ 16 fl. oz./470 ml), enough for four ½-cup (4 oz.) servings
2 3/4 oz (82 g) premium dark couverture, preferably Valrhona Caraïbe 66% cacao
2/3 oz (19 g) premium milk couverture, preferably Valrhona Jivara 40% cacao
2 tablespoons (28 g) 2% milk
¼ cup plus 1 1/4 teaspoons (64 g) heavy whipping cream
3 large (90 g) egg whites
1 tablespoon (13 g) firmly packed dark brown sugar, preferably Organics brand
Make the base. Roughly chop the chocolates together and place them in a small mixing bowl. Place the milk and cream in a small saucepan, combine briefly with a whisk, and set over medium heat. Heat to 176°F/80°C (a skin will form on the surface, and the cream will be on the verge of simmering).
Pour the hot liquid over the chocolate and start whisking vigorously in the center with a whisk held vertically, bringing the chocolate in from the sides as necessary; this quickly creates an emulsion. Whisk until the chocolate is fully melted. Set aside to cool slightly, about 2 minutes, stirring on occasion and scraping the sides of the bowl.
Make the meringue. When the chocolate mixture has cooled to about 94°F/34°C (it will feel just below body temperature), add the egg whites to a very clean mixing bowl and immediately add the brown sugar. Using an electric mixer, start beating the egg whites on low speed (speed 1 on a handmixer; speed 2 on a stand mixer) for 1 minute 45 seconds (the mixture will be very foamy across the top). Increase the speed to setting 4 if using a stand mixer (or maintain at setting 1 for a handmixer) and beat for 2 minutes and 45 seconds. The meringue should look silky. You want to create even microbubbles in the meringue, not large visible bubbles. When completed, the meringue will hold medium-stiff peaks that curve down slightly at the tip when the beater is held horizontally. Do not overbeat the egg whites or else they risk breaking apart.
Compose the mousse. When the chocolate mixture has cooled to 86°F/30°C (it will feel cool to the touch but not be thickening), delicately fold one-third of the egg whites into the chocolate mixture until mostly incorporated, then add the remaining egg whites at once. Start folding gently through the center and around the edge of the bowl, scraping the sides and bottom of the bowl clean on occasion while moving the spatula through the center and around the edge in a while turning the bowl in the opposite direction, just until the mixture is homogenous. Do not overfold or you risk deflating the mousse. The mousse should maintain a pourable consistency but be light and very airy.
Immediately and gently pour the mousse into a large measuring cup with a spout, then gently pour the mousse into serving dishes. Chill until set, for a minimum of 30 minutes (if in a hurry) but ideally for several hours until fully set. Keep refrigerated, covered loosely with plastic wrap. The mousse will keep for several days if prevented from drying out.
Once fully chilled and set, decorate by adding textural components: a pile of chocolate curls, crushed caramelized cocoa nibs, a light dusting of Dutch-process cocoa powder, and/or a few sea salt flakes, if desired. Other topping options are toasted hazelnut halves, toasted sliced almonds, chocolate pearls, and/or crispy crêpe cookies.
Set the mousse out at room temperature for about 20 minutes before serving to soften somewhat yet maintain a slight chill.