Preheat oven to 350 degrees F.
Grease three 9-inch cake pans, line them with parchment rounds and flour the sides.
Put the chocolate in a heat proof bowl and pour hot water over it. Let it sit for a couple of minutes, then stir until melted and smooth.
Whisk or sift together the (already sifted once) cake four, baking soda and salt.
In an electric mixer on medium speed, cream the butter with the sugar until light and fluffy. Beat in the egg yolks, one at a time, followed by the vanilla. Reduce mixer speed to low and stir in the melted chocolate. By hand or using lowest speed, add flour mixture alternately with buttermilk (beginning and ending with the buttermilk).
Beat the egg whites to almost stiff peaks in a separate bowl. Fold about 1⁄2 cup of the batter into the egg whites to lighten it up, then fold the egg whites into the batter.
Divide the batter among the three cake pans and bake on middle and lower racks for 35 minutes or until a toothpick inserted in the center comes out with moist crumbs.
Cool the cakes for a few minutes and unmold. Remove the parchment, invert cakes back to upright and cool on racks.