1. Preheat oven to moderate 350º. Grease bottom and side of a 10 inch springform pan; dust lightly with flour; tap out any excess.
2. Sift flour and baking powder onto wax paper.
3. Pour boiling water over butter and chocolate in a large bowl; let stand 5 minutes until melted; stir to blend. Stir in sugar and vanilla. Beat in egg yolks, one at a time, blending thoroughly.
4. Combine sour cream and baking soda in a bowl; beat into chocolate mixture. Stir in flour mixture, blending thoroughly.
5. Beat egg whites in a small bowl with electric mixer until soft peaks form. Stir a little of the beaten egg whites into batter to lighten. Fold remaining egg whites into chocolate mixture until not streaks of white remain. Pour into prepared pan.
6. Bake in preheated moderate oven 350º for 45 minutes or until a small knife inserted near center comes out clean. Cool cake in pan on wire rack 10 minutes; loosen spring from side; remove side of pan; cool cake completely. Frost with chocolate frosting; Decorate with crystallized violets, if you wish. Let set before cutting cake.
1. Combine chocolate, heavy cream, butter, vanilla and sugar in a medium size sauce pan.
2. Heat slowly, stirring constantly, until butter and chocolate have melted. Remove from heat; beat until mixture thickens slightly, but is still runny. Cool slightly. Frost