Preheat oven to 350 °F (175 °C). Grease and line two 9-inch round cake pans with parchment paper.
In a large bowl, whisk together flour, cocoa powder, sugar, baking soda, baking powder, and salt.
Add eggs, buttermilk, vegetable oil, and vanilla extract. Beat on medium speed for 2 minutes until smooth.
Stir in hot water or coffee until fully combined. The batter will be thin — this is expected.
Divide batter evenly between the two prepared pans.
Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.
Cool in pans for 10 minutes, then transfer to a wire rack to cool completely.
Frost with chocolate buttercream frosting, decorate as desired, and serve.