Grease and flour a 6-inch heart-shaped or round springform pan; line bottom with parchment or wax paper. Set aside.
Melt butter and chocolate in a heavy saucepan over low heat, stirring constantly, until smooth. Remove from heat; let stand 2 to 3 minutes. Pour into a mixing bowl.
Stir together sugar and flour. Gradually add to chocolate mixture, beating at medium speed with an electric mixer until blended. Add egg yolks, beating well after each addition.
Beat egg whites and salt at high speed with electric mixer until stiff peaks form. Gently fold egg whites into chocolate mixture. Pour into prepared pan.
Place cakepan in a 13- x 9-inch pan, and add hot water to larger pan to a depth of 1 inch.
Bake at 350° for 1 hour or until center appears firm. Remove pan from water, and cool on wire rack 15 minutes. Invert cake onto wire rack, and peel off paper. Cool completely.
Spread Amaretto Buttercream Frosting over top of cake, and freeze 15 minutes.
Place cake on wire rack in a 15- x 10-inch jellyroll pan. Pour French Chocolate Glaze over cake, allowing it to drip down sides. Spread glaze over sides of cake with a spatula. Spoon excess glaze from pan into a heavy-duty zip-top plastic bag; seal. Snip a tiny hole in 1 corner of bag, and drizzle chocolate over cake. Let cake stand at least 1 hour before serving. Garnish, if desired.