Preheat oven to 350 degrees F. Cover bottom of three 9-inch round pans with parchment paper. Grease well.
Chop chocolate and mix with water in a small microwave-safe bowl. Microwave on HIGH 1-1/2 - 2 minutes, until chocolate is mostly melted. Stir until smooth.
In small bowl, whip egg whites with electric mixer until stiff peaks form. Set aside.
In large mixing bowl, beat together butter and sugar until light and fluffy. Add egg yolks and beat well. Blend in chocolate and vanilla.
Add dry ingredients (flour, salt, soda) alternating with buttermilk, beating well after each addition.
Add whipped egg whites, folding gently with a spatula until completely combined. Pour into prepared pans.
Bake 30 minutes until toothpick inserted into centers comes out clean. Immediately run knife around the cakes to loosen. Cool cakes in pans for 15 minutes. Remove from pans to wire rack to cool completely