Top-secret Chocolate Mousse

From: Dorie Greenspan’s around my french table 

3.5 ounces bittersweet chocolate, coarsely chopped
3 large eggs, separated, at room temperature
pinch of salt
1.5 teaspoons sugar

Whipped cream or creme fraiche, for serving (optional)

Gently melt the chocolate in a heatproof bowl over a saucepan of simmering water or in a microwave oven over medium power.
If necessary, transfer the chocolate to a bowl that can hold all of the ingredients. Using a whisk, stir the egg yolks into the chocolate one at a time.
In the bowl of a stand mixer fitted with the whisk attachment or in a bowl wit a hand mixer, beat the egg whites with the salt until they start to form peaks. Beating all the while, gradually add the sugar. Continue to beat until the whites are shiny and hold medium-firm peaks.
Spoon about 1-quarter of the whites over the melted chocolate and stir with the whisk until the mixture is almost smooth. (Stirring in a bit of the whites lightens the chocoalte and makes the next step easier.) Spoon the rest of the whites over the chocolate and using the whisk or a large rubber spatula, very carefully fold them in. Be as thorough as you can without overworking the mixture - it’s better to have a few white streaks than to beat the bubbles out of the mousse by overmixing.
Spoon the mousse into a serving bowl or individual bowls and serve it now or cover it and keep it in the refrigerator until you’re ready for dessert. Serve with whipped cream or creme fraiche if you like.