Separate the eggs while they're cold, and place the whites in the bowl of a stand mixer and allow them to come to room temperature.* Place the yolks in a medium sized heat proof bowl.
Fill a large sauté pan with 2 inches of water. Place over the stove top and bring to a simmer. Add 1/4 cup of the sugar, cooled espresso, and sea salt to the egg yolks and whisk to combine. Place in the bowl of gently simmering water, and whisk constantly until the yolks almost double in volume, and reach a temperature of 160 degrees, about 1 minute.
Remove the bowl from heat and add the chopped chocolate, whisking until melted and fully incorporated. Set aside and allow the mixture to come to room temperature, whisking occasionally.
Once the egg whites reach room temperature, fit a stand mixer with the whisk attachment and beat at medium speed until frothy. With the mixer still running, add the remaining 2 tablespoons of sugar a little at a time. Increase the speed to medium high and whip until stiff, glossy peaks form. When the wire whip is dipped into the whites, turned right side up and the whites stand straight up at the tip, they're ready.
In a separate bowl, beat the heavy cream until it forms stiff peaks. Reserve 1/2 cup for garnish, and store covered in the refrigerator until ready to use.
Fold the egg whites into the chocolate mixture gently, then fold in the remaining whipped cream until just incorporated. Pour into wine glasses, coffee mugs, or ramekins, cover, and chill a minimum of 2 hours, and up to one day in advance. Whisk the reserved 1/2 cup whipped cream briefly before serving, and garnish the mousse with the cream and grated chocolate. Enjoy!
Notes
*I used instant espresso prepared according to package instructions for the coffee.
*To bring the egg whites to room temperature more quickly, place in a heat proof bowl and heat briefly in the pan of simmering water just until lukewarm.
Recipe technique adapted from Bon Appetit's Classic Chocolate Mousse and Tante Marie Cooking School in San Francisco, CA. Mousse recipe by Flavor the Moments.