Preheat the oven to 350 degrees F. Line with parchment paper and grease 3 (8-inch) round cake pans or 2 (9-inch). Set aside.
In a glass measuring cup microwave the water until boiling. Stir in the chocolate until completely melted and smooth. Set aside.
In a medium-size bowl whisk together the flour, baking soda, and salt. Set aside.
In a large mixing bowl beat the butter and sugar together with an electric mixer on medium speed until light and fluffy. About 3-4 minutes. Add the eggs (whole eggs, not separated), one at a time, beating well after each addition.
Stir in the melted chocolate mixture and vanilla extract.
Beat in the flour mixture in thirds alternating with the buttermilk, beating after each addition until smooth.
Divide the cake batter evenly into all three prepared cake pans—about 2 1/2 cups of batter for each pan.
Bake for 30 minutes or until cake springs back when lightly pressed in the center. Transfer the cakes to a wire rack to cool for at least 15 minutes in the pan. Remove the cakes from the pan back to the wire rack to cool completely before frosting.