Preheat the oven to 350°F (175°C). Grease and line two 9-inch round cake pans.
In a large mixing bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add buttermilk, oil, eggs, and vanilla. Beat until well combined.
Carefully stir in the hot water until the batter is smooth. Add black food coloring to deepen the color.
Divide the batter evenly between the pans.
Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean.
Let the cakes cool in the pans for 10 minutes before transferring to a wire rack to cool completely.
Frost with your favorite cream cheese or buttercream frosting and serve.