1 cup semi-sweet chocolate (roughly chopped or chocolate chips, plus more to garnish)
6 eggs (yolks and whites separated)
2 tablespoons butter
4 tablespoons powdered sugar
whipped cream (to serve)
unsweetened cocoa powder (sifted, to serve)
To build a double boiler, fill a medium saucepan with a couple inches of water and place over medium heat. Set a heat-proof bowl over the pot. Place the chocolate in the bowl of the double boiler and melt the chocolate.
Remove the chocolate from the heat. Add the butter, while whisking constantly to emulsify. Add the yolks one at a time, whisking constantly until the mixture is homogenous.
In a standing mixer with a whisk attachment, whip the egg whites until frothy. Turn the machine off, add the powdered sugar, and whip on medium speed to stiff peaks.
With a rubber spatula, gently fold the meringue into the chocolate mixture, until completely incorporated.
Transfer to a serving bowl and chill for 4 to 6 hours.
Serve mousse in individual bowls or ramekins. Top with a dollop of whipped cream, cocoa powder, and shaved chocolate.