Preheat Oven:
Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
Mix Dry Ingredients:
In a large bowl, sift together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Combine Wet Ingredients:
In another bowl, whisk together buttermilk, oil, eggs, and vanilla extract until smooth.
Combine Batter:
Gradually add wet ingredients to the dry ingredients, mixing until just combined. Slowly add hot water or coffee, mixing until smooth. Batter will be thin—this is normal.
Bake:
Divide batter evenly between prepared pans. Bake for 30–35 minutes or until a toothpick inserted in the center comes out clean.
Cool:
Let cakes cool in pans for 10 minutes, then transfer to a wire rack to cool completely.
Prepare Frosting:
Beat butter until creamy. Add cocoa powder, powdered sugar, milk, and vanilla extract. Beat until smooth and fluffy. Adjust consistency with milk if needed.
Assemble Cake:
Place one cake layer on a serving plate. Spread frosting evenly over the top. Place second cake layer on top and frost the top and sides.
Serve:
Slice and serve as desired. Decorate with chocolate shavings, sprinkles, or fresh berries if desired.