- 6 ounces bittersweet chocolate, finely chopped
- 1/3 cup whole milk
- 1 large egg yolk
- 4 large egg whites
- 2 tablespoons sugar
Melt the chocolate in a bowl over (not touching) simmering water or in the microwave oven. (If you want to melt the chocolate in the microwave, set it to 30 seconds and stir thoroughly, then every 15 seconds thereafter, until melted.) If necessary, transfer the chocolate to a bowl that is large enough to hold all of the ingredients. Keep the chocolate on the counter until needed. The chocolate should still feel warm to the touch when you're ready to use it.
Bring the milk to the boil, then pour it over the chocolate. Using a small whisk, gently blend the milk into the chocolate. Add the egg yolk and whisk it into the chocolate, again working gently; stop when the yolk is incorporated.
In a mixer fitted with the whisk attachment, beat the egg whites on medium speed just until they hold soft peaks. Increase the speed to medium-high and gradually add the sugar. Continue to beat the whites until they are firm but still glossy. Scoop 1/3 of the whites out of the bowl onto the chocolate mixture. Working with a whisk, beat the whites into the chocolate to lighten the mixture. Now, with either the whisk or a large flexible rubber spatula, delicately but thoroughly fold the rest of the beaten whites into the chocolate.
Turn the mousse into a large serving bowl—glass is very nice for this dessert—or into individual cups, and refrigerate for 1 hour to set.
On storage: Although the texture of the mousse will be lighter if you serve it shortly after chilling, it can be covered and kept in the refrigerator for up to two days. It will be just as delicious, if a little denser.