Enjoy this dessert at room temperature if you like a creamy texture similar to that of a soufflé, or serve the cake cold if you prefer a dense, fudgey texture.
03Stir butter and 1/2 cup whipping cream in heavy large saucepan over medium heat until butter melts and mixture simmers. Remove from heat. Add chocolate and whisk until smooth. Stir in both almond mixtures. Cool slightly.
04Using electric mixer, beat egg whites in large bowl until soft peaks form. Gradually add 1/3 cup sugar and beat until stiff peaks form. Beat egg yolks in another large bowl until very pale and thick, about 5 minutes. Gradually beat chocolate mixture into egg yolks. Fold in egg whites in 3 additions.
05Pour batter into prepared pan. Bake cake until sides crack and puff and tester inserted into center comes out with moist batter attached, about 35 minutes. Transfer cake to rack. Cool cake to room temperature, about 2 hours (center will fall slightly as cake cools.) (Can be prepared 4 days ahead. Cover and refrigerate.)
06Beat chilled cream, amaretto and remaining 2 tablespoons sugar in large bowl until soft peaks form.
07Run small sharp knife around pan sides to loosen cake. Release pan sides. Dust cake with powdered sugar. Sprinkle toasted almond slices around top edge of cake. Serve chilled or at room temperature with whipped cream.