Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add buttermilk, vegetable oil, eggs, and vanilla extract. Mix until smooth.
Slowly pour in hot water or coffee while mixing until the batter is well combined.
Divide the batter evenly between the prepared pans.
Bake for 30-35 minutes, or until a toothpick inserted comes out clean.
Let cakes cool in pans for 10 minutes, then transfer to a wire rack to cool completely.
For the buttercream, beat softened butter until creamy.
Gradually add powdered sugar, vanilla extract, and heavy cream or milk until smooth and fluffy.
Frost the cooled cake with the vanilla buttercream.