The rich, deep chocolate flavor contrasts with the incredibly light and airy texture of this classic French dessert. Use your favorite high-quality dark chocolate for the best results.
Place egg whites in a large bowl, and refrigerate, covered, until ready to use. Whisk together egg yolks and granulated sugar in a medium-size heatproof bowl until mixture is light yellow and creamy; set aside.
Bring milk and cream to a boil in a small saucepan over medium-high. Place a kitchen towel under bowl with egg yolk mixture to secure bowl. Whisking constantly, slowly pour hot milk mixture into egg yolk mixture until fully combined. Transfer mixture back to pan. Reduce heat to medium, and cook, stirring constantly, until mixture coats back of a spoon and leaves a path when a finger is drawn across spoon, 3 to 4 minutes. (Do not let it simmer.) Remove from heat; stir in chocolate until smooth. Pour mixture through a fine wire-mesh strainer into a bowl; discard any solids. Refrigerate, uncovered, until chilled, about 2 hours.
Bring a medium skillet filled with water to just below a simmer (about 180°F) over medium. Whisk chilled egg whites until soft peaks form, 3 to 4 minutes. Slowly whisk in powdered sugar, 1 tablespoon at a time. Whisk mixture until stiff peaks form, about 1 minute.
Line a baking sheet with paper towels. Use a large oval spoon to scoop about 2/3 cup meringue. With a second large oval spoon, gently transfer scooped meringue from spoon to spoon, shaping meringue into a smooth, oval-shaped quenelle. Gently drop quenelle into prepared 180°F water. Repeat to make 2 more quenelles. Cook until set and puffed up, about 4 minutes, flipping once halfway through. Using a slotted spoon, gently lift quenelles from water, and transfer to prepared baking sheet. Let cool 5 minutes. Repeat shaping and poaching to make a total of 6 quenelles.
To serve, divide chocolate sauce among 6 shallow bowls; place 1 quenelle in center of each bowl. Sprinkle with chopped pistachios, and serve.
Chocolate sauce may be made up to 3 days ahead. Cover and chill until ready to serve.