Preheat oven to 350F (175C). Grease and flour a 9x13 inch baking pan.
In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add buttermilk, oil, eggs, and vanilla extract to the dry ingredients. Beat with an electric mixer on medium speed for 2 minutes.
Gradually add the boiling water to the batter, mixing until combined. The batter will be thin.
Pour the batter into the prepared pan and bake for 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Let the cake cool completely in the pan before frosting.
To Prepare the Frosting:
In a large bowl, beat the softened butter with an electric mixer until creamy.
Gradually add the powdered sugar and cocoa powder, alternating with the milk, beating until smooth and creamy.
Stir in the vanilla extract.
Frost the cooled cake evenly.
Slice, serve, and enjoy!
Tips & Substitutions:
For a richer flavor, substitute coffee for the boiling water.
To make the cake gluten-free, use a 1:1 gluten-free all-purpose flour blend.
*For a vegan version, substitute the buttermilk with a mixture of plant-based milk and lemon juice, use flax eggs instead of chicken eggs, and ensure the frosting ingredients are also plant-based.