Step 1

Place the butter in a medium microwave-safe bowl. Break the chocolate into small pieces directly into the bowl. Microwave it in 20-second intervals, stirring between each bout of heat, until the chocolate is about 75% melted. Stir, allowing the residual heat in the bowl to melt the chocolate completely. (Alternatively, place the chocolate and butter in a heatproof bowl and place over a saucepan containing about 1 inch of barely simmering water. Stir with a wooden spoon until the chocolate is melted and the mixture is smooth.) Let the mixture cool for a few minutes, then whisk in the egg yolks one at a time, mixing until smooth after each addition. Set aside.

Step 2

In the bowl of a stand mixer or electric hand mixer, beat the egg whites on medium-high speed until foamy. Add the cream of tartar and beat until soft peaks form (the peaks should be just starting to hold, and will melt back into themselves after a second). Gradually beat in 1/4 cup of the sugar and continue beating until stiff peaks form (the peaks will stand straight up when the beaters are lifted from the mixture). Using a large rubber spatula, fold the egg white mixture into the chocolate mixture until uniform. Set aside.

Step 3

In another bowl, beat the heavy cream on medium-high speed until it begins to thicken up. Add the remaining 2 tablespoons of sugar and the vanilla and continue beating until the cream holds medium peaks (when you lift the beaters or whisk out of the bowl, the peaks will slightly droop down, but they won't lose their shape entirely). Fold the whipped cream into the chocolate mixture. Be sure it is fully incorporated but don't mix any more than necessary. Divide the mousse between 6 individual glasses, cover, and chill until set, at least 2 hours.

Step 4

Up to a few hours before serving, whip the cream until it begins to thicken up. Add the sugar and whip to medium peaks. Dollop the whipped cream over the mousse and top with chocolate shavings.

Step 5

Make-Ahead Instructions: Mousse can be made up to 1 day ahead of time. Cover with plastic wrap and keep chilled in the refrigerator. Add whipped cream topping and chocolate shavings up to a few hours before serving.

Step 6

Note: Chocolate mousse is made with raw eggs. If that is a concern, try to find pasteurized eggs. Pasteurized eggs have been briefly exposed to heat to destroy any potential bacteria. Whole Foods usually carries them but call your local store to be sure before making a special trip. (Note: eating raw eggs is not advisable for pregnant women, babies, the elderly, or anyone with a compromised immune system.)