How to Make It
Preheat the oven to 350°. Butter and flour a 10-by-5-by-4-inch loaf pan. In a medium bowl, sift the flour with the cocoa powder, baking powder, baking soda and salt. In a large bowl, beat the butter until creamy. Add the sugar and beat until light and fluffy. Add the eggs, 1 at a time, beating for 1 minute between additions. Add the dry ingredients and beat at low speed, scraping down the side of the bowl. Add the water in a thin stream and beat until the batter is silky and fluffy. Scrape the batter into the loaf pan and smooth the top. Bake for 1 hour and 5 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes before transferring it to a rack to cool completely. Clean the loaf pan.
In a saucepan, combine 1 cup of the cream with the egg yolks and 1/2 cup of the sugar and cook over moderately low heat, stirring constantly, until the custard coats the back of the spoon, about 10 minutes. Remove the pan from the heat, add the chocolate and gently stir until melted and smooth. Let cool to room temperature.
In a medium bowl, whip the remaining 1 cup of cream until firm; chill. In a large bowl, beat the egg whites to soft peaks. Gradually add the remaining 1/2 cup of sugar. Beat until firm, glossy peaks form. Fold the egg whites into the chocolate mixture, then fold in the whipped cream. Refrigerate the chocolate mousse.
Line the loaf pan with plastic wrap, leaving at least 6 inches of overhang all around. Using a long serrated knife, slice off the top of the cake, so that you have a neat rectangular loaf. Cut the loaf horizontally into five 1/2 -inch-thick slices. Line the bottom of the loaf pan with the bottom slice of cake and line the 2 long sides with 2 more slices. Cut 1 of the remaining cake slices to fit the short ends and wedge the pieces into the pan. Gently press all the edges together. Pour the mousse into the loaf pan and top with the remaining cake slice. Fold up the plastic wrap and freeze the cake until firm, at least 5 hours or preferably overnight. Eat the cake scraps.
Let the cake stand at room temperature for 15 minutes before unwrapping and unmolding; then cut it into 1-inch-thick slices. Transfer the slices to plates and serve right away or let stand at room temperature until thawed.