Decadent Chocolate Cake

Preheat oven to 350 degrees. Grease and sprinkle a bit of flour into a 9-inch spring-form pan. Knock out excess. Pour boiling water over chocolate and butter; let stand until melted. Use a mixer and stir in vanilla and sugar, then whisk in egg yolks, one at a time, blending well after each addition. In a separate bowl, mix baking soda and sour cream and whisk into chocolate mixture. Sift flour and baking powder together and add to batter, mixing thoroughly. Beat egg whites until stiff but not dry. Using the mixer, stir a quarter of the egg whites thoroughly into the batter. Scoop remaining egg whites on top of the batter and gently fold together. Pour batter into the prepared pan. Set on the middle rack of the oven and bake for 40-50 minutes, or until the edges have pulled away from the sides of the pan and a cake tester inserted into the center comes out clean. Cool in pan for 10 minutes; un-mold and cool completely before frosting. Can also top with nuts and/or fruit.