This rich chocolate mousse cake is made with chocolate ganache, which includes finely chopped bittersweet chocolate.
Line a 10-by-4 1/2-by-3-inch loaf pan with plastic; let excess hang over sides of pan. Line pan with chocolate genoise: Cut one 9-by-4-inch rectangle to fit bottom and two 9-by-3-inch rectangles to fit sides of loaf. Cut four 2 1/4-by-4 1/2-inch pieces; use two to fit each end of the loaf; set other two aside. Cut another 10 1/2-by-5-inch rectangle to fit top; set aside.
Place chocolate in a large heat-proof bowl or the top of a double boiler set over a pan of barely simmering water; heat until melted. Set aside.
Combine yolks and sugar in a medium bowl; whisk until pale yellow. Add flour and cornstarch; whisk to combine. Place milk in a small saucepan; bring to a boil.
Gradually pour milk into yolk mixture, whisking constantly. Return mixture to pan; set over medium-low heat. Cook, whisking constantly, just until mixture comes to a boil. Remove from heat; immediately pass through a fine sieve into melted chocolate. Use a rubber spatula to combine; set aside to cool. When almost cool, whip the heavy cream to soft peaks, and fold into the chocolate mixture.
Spoon chocolate mousse into the prepared loaf, and top with the reserved genoise. Wrap the excess plastic over the loaf, and place in the freezer overnight.
Unmold cake onto a wire rack set over a rimmed baking sheet. Pour ganache over cake, coating completely. Allow ganache to set for about 10 minutes. Scrape excess ganache from baking sheet back into the bowl. Pour a second coating of ganache over cake. If ganache gets too thick, it can be rewarmed slightly over simmering water. Transfer cake to a serving platter; refrigerate for 1 hour or until ready to serve. Garnish with edible roses and rose petals just before serving. Thinly slice with a serrated knife. Serve with creme anglaise.