Back when I was a pastry chef I made this almost every day, frequently doing a double or even triple batch. It isn't piped into individual serving bowls; it's molded in a loaf pan. That makes for easier service and doesn't require any special dishes. Typically, I served it on a bed of Crème Anglaise with a few berries if they were in season. This recipe will make a loaf that will serve 12-16 guests.
1 lb. bittersweet chocolate (63% cacao)
8 oz. unsalted butter
¼ teaspoon cinnamon (optional)
1 shot espresso (optional)
pinch of salt (optional)
4 egg yolks
8 egg whites
pinch cream of tartar (optional)
1 cup heavy cream
Gently melt the chocolate and butter, then stir in the optional cinnamon, espresso, and salt. Separate four eggs, stirring the yolks into the melted chocolate and reserving the whites. Then separate four more eggs, reserving the yolks for crème anglaise or another recipe. Whip the cream to stiff peaks, then whip the egg whites, optionally adding a pinch of cream of tartar, to soft peaks. Gently fold the whipped cream into the chocolate mixture. Give the whites a finally touch-up with a whisk, then fold into the mousse.
Line a 1-pound loaf pan generously with plastic wrap, then pour the mousse into the pan, tap it on the counter to remove air bubbles, fold the plastic over the mousse to cover, then refrigerate. It will usually set within an hour, but tastes best if you leave it overnight.
If you've done a good job of whipping the whites and cream, and folded them gently, you'll end up with an extra portion that just won't fit in the pan. Share it with someone you really, really love, or just don't tell anyone about it and keep it all to yourself.
When you're ready to serve, wrap a piece of cardboard or a small cutting board in plastic. Open the plastic on the loaf pan to expose what will become the bottom of the mousse. Set the top side of the board you're unmolding onto against the exposed mousse, invert, remove the pan, and carefully peel off the plastic wrap to expose a very dense, very chocolately loaf of absolutely heavenly goodness.
To serve, slice with a palette knife dipped into hot water. A flat spatula or second palette knife, also dipped in hot water, will be useful to transfer the slice to the serving plate.