Chocolate Mousse Cake

Butter two 9" round cake pans and line the bottoms with wax paper, then butter the paper. Dust the pans with flour and knock out the excess. Preheat the oven to 350 degrees. In the top of a double boiler, heat the chocolate over hot water, stirring, until just melted. Remove the pan from the heat and stir in the butter, one tablespoon at a time. Continue to stir the mixture until smooth. In the bowl of the mixer, beat the egg yolks until combined. Add 3/4 cup of the sugar, a little at a time, and continue to beat the mixture until it falls in a ribbon when the beater is lifted. Beat in the melted chocolate mixture, the liqueur and the orange pinch of salt until frothy. Add the cream of tartar and beat the whites until they hold soft peaks. Add the remaining 1/4 cup sugar, a little at a time, and beat the whites until they are stiff. Sift the flour with the remaining 1/4 teaspoon salt onto a sheet of wax paper. Stir one fourth of the whites into the batter. Fold in the remaining whites and sift and fold the flour mixture in batches into the egg mixture until just combined. Pour the batter into the cake pans, smoothing the tops, and bake the layers in the middle of the oven for 30 to 35 minutes, or until a cake tester inserted into the centers comes out clean. Let the cakes cool in the pans on racks for 5 minutes, invert the cakes onto the racks, and remove the wax paper carefully. Let the cakes cool completely. The cakes form a thin crust that will flake off. In a small saucepan bring the cream just to a simmer. Remove the pan from the heat, add the chocolate and let the mixture stand, covered, for 5 minutes. Stir in the liqueur and continue to stir the ganache until smooth and tepid. Strain through a fine sieve into a small pitcher or a bowl with a lip. Cut an 8" cardboard round and put one teaspoon of the mousse in the center of it, then set one of the cake layers, bottom side up on the cardboard. Cover the cake with some of the mousse, smoothing it into an even layer, and top it with the remaining layer, bottom side up. Cover the top and the sides of the cake with the remaining mousse, smoothing it with a spatula, and chill the cake until it is cold. Set the cake on a rack over a jelly roll pan and pour the creme ganache over it, smoothing it with a spatula to completely cover the top and sides of the cake. Let the cake stand at room temperature for 10 minutes and scrape any excess chocolate glaze from the jelly roll pan back into the saucepan. Heat the excess ganache, stirring, until smooth, cool it to tepid, and pour it over the cake, smoothing it with a spatula over the top and sides of the cake. Chill the cake until the glaze is set. Transfer the reserved chocolate mixture to a pastry bag. Arrange orange slices, rounded sides up on the top of the cake, so that one end of the slice is toward the center and the other is toward the edge of the cake. Pipe the chocolate mixture along the base on each side of the orange slices. Arrange the remaining cream decoratively around the bottom edge of the cake. Let the cake stand at room temperature for at least 15 minutes before serving.