Preheat your oven to 350°F (175°C) and prepare two 9-inch round baking pans by greasing and flouring them.

In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt until well combined.

Add the buttermilk, warm water, vegetable oil, eggs, and vanilla extract to the dry ingredients, then mix with an electric mixer on medium speed for about 2 minutes until the batter is smooth.

Divide the batter evenly between the prepared baking pans and bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.

Allow the cakes to cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely.

In a large bowl, beat the softened butter with an electric mixer until creamy.

Gradually add the powdered sugar and cocoa powder, alternating with milk, beating until smooth and well combined.

Stir in the vanilla extract and beat until the frosting is light and fluffy.

Once the cakes are completely cool, place one layer on a serving plate or cake stand.

Spread about one-third of the frosting over the top of the first cake layer, then carefully place the second cake layer on top.

Frost the top and sides of the entire cake with the remaining frosting, smoothing it out with an offset spatula or knife.