DARK CHOCOLATE MOUSSE

Steps:

• In a double boiler, melt chocolate and butter together, stirring occasionally with a whisk. (If you don't have a double boiler, use a heatproof bowl set over a pot of barely simmering water, making sure the bowl isn't touching the water.) Meanwhile, in a stand mixer fitted with a whisk attachment, whip egg yolks and 2 tablespoons sugar at high speed, 2 minutes, until it reaches the "ribbon stage"-i.e., the batter resembles a ribbon when you lift the whisk.
• Pour warm chocolate mixture into a large mixing bowl. Using a rubber spatula, quickly fold the egg yolk mixture into the chocolate, stopping after the mixture is combined three-fourths of the way. Set aside.
• In a stand mixer fitted with a whisk attachment, whip the egg whites and salt at medium speed until the mixture is frothy and forms soft peaks, 1 minute. With the mixer still running, slowly add the remaining 4 tablespoons of sugar in a steady stream, then turn mixer to high and whip until medium peaks form, 1 more minute.
• Fold a quarter of the egg whites into the chocolate mixture to lighten, then fold in the remaining egg whites gently but thoroughly using the following method: Look at the mixing bowl like a clock-the top is 12 o'clock, the bottom 6 o'clock. Place the rubber spatula, point side down, into the bowl at 12 o'clock. Stir the mousse clockwise around the edge of the bowl to 9 o'clock, while turning the bowl counterclockwise. Repeat, about 6 times, until there are no more visible egg whites.
• In a stand mixer fitted with a whisk attachment, whip the vanilla extract and heavy cream at medium speed, 30 seconds. Then turn to high and whip until medium peaks form, 1 more minute. Fold whipped cream into mousse gently but thoroughly, using the clock method from Step 4. Makes 4 cups mousse.
• Assembly: Evenly divide mousse into 8 individual serving bowls (an ice cream scoop works well for this). Chill in the refrigerator to set, 2 hours or up to overnight. Serve with additional whipped cream dolloped on top.