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Add the flour, granulated sugar, cocoa powder, baking powder, baking soda and salt to the bowl of a standing mixer fitted with the whisk attachment. Turn on the mixer and allow the dry ingredients to blend for a full minute. Turn the mixer up and, while the mixer is running, add the eggs, one at a time, the buttermilk, warm water, melted butter and vanilla extract. Beat until combined, about 2 minutes. The batter will be thick.
Evenly divide the batter between the prepared pans. Tap the bottom of the cake pans on the counter to release any air bubbles. Bake until a toothpick inserted into the center of the cake comes out clean, about 30 minutes. Cool in the pans for 10 minutes, and then turn out onto a wire rack to cool completely before frosting.
For the frosting: Add the butter to a medium saucepan over medium heat. Add the chopped chocolate and stir to melt. Remove from the heat. Stir in the cocoa powder and vanilla extract. Using an electric hand mixer, beat in the confectioners' sugar. Slowly beat in the sour cream. Let cool for 20 minutes.
Frost the cake by first spreading the frosting between the layers. Then top with cake, and continue frosting the top and the sides.