Preheat oven to 350. Grease & flour two 9 in round cake pans.
Combine cocoa powder &boiling water in a bowl & stir until chocolate is dissolved...about 30 seconds.
Combine sugar & butter in large bowl. Beat with electric mixer till light & fluffy...1-2 minutes. Add cocoa mixture & beat 1 min. longer.Beat in sour cream, vanilla & baking soda. Add flour & salt. Mix 1 minute more.
Beat egg whites in medium bowl till stiff peaks form...2-3 minutes. Gently fold into batter.
Spoon batter into prepared pans. Bake till toothpick inserted in center comes out clean...25-28 minutes. Transfer to wire racks to cool. Cut each cake into 2 even layers for total of 4 layers.Spread frosting between the layers & over top & sides of cooled cake.