Line a 23 x 13-cm (9 x 5-inch) loaf pan with parchment paper letting the paper hang over 2 opposites sides. In the sheet cake, cut a 20 x 10-cm (8 x 4-inch) rectangle and place at bottom of the pan. Cut two 23 x 5-cm (9 x 2-inch) pieces and cover the walls on both sides. Spread the mousse in the pan. Cover with a 23 x 13-cm (9 x 5-inch) piece of sponge cake. Cover with plastic wrap and refrigerate for 4 hours.
