Made like a chocolate mousse, this rich loaf is firm enough to be cut into slices and served with a sauce. The terrine can be tightly wrapped and frozen for up to 1 month. Let stand at room temperature for 1 hour before slicing and serving.

Lightly oil an 8 ½ × 4 ½-inch loaf pan and line with plastic wrap, pressing it firmly into the corners and against the sides.

Melt in a large heatproof bowl set over a pan simmering water, stirring until smooth:

Set the bowl aside (keep the pan of hot water). Whisk together in a large heatproof bowl:

Place the bowl over the simmering water and whisk constantly until thick, puffy, and shiny, like marshmallow sauce, at least 10 minutes. Remove from the heat and whisk in the chocolate mixture until thoroughly combined. The mixture will deflate a bit. If there is any sign of separation, whisk in cold water, 1 teaspoon at a time, until the mixture is smooth. Beat in a large bowl on medium speed until soft peaks form:

Gradually sprinkle in:

Increase the speed to high and beat until the peaks are stiff but not dry. Fold one-third of the egg whites into the chocolate mixture, then fold in the remaining whites. Turn the mixture into the prepared pan and cover with plastic wrap. Refrigerate for at least 8 hours. Remove the plastic from the top, invert the terrine onto a serving plate, and peel off the plastic liner. Cut crosswise into slices and serve with: