Preheat oven to 350F (175C). Grease and flour a 9x13 inch baking pan.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the 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 just combined. The batter will be thin.
Pour the batter into the prepared pan and bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out with a few moist crumbs.
Let the cake cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely.
Frosting Instructions
In a large bowl, beat the softened butter with an electric mixer until smooth and creamy. Gradually add the powdered sugar and cocoa powder, alternating with the milk, until the frosting reaches desired consistency. Stir in the vanilla extract. Frost the cooled cake evenly. Cut into squares and serve.
Optional Tips:
For a richer chocolate flavor, substitute the boiling water with hot coffee. To prevent the cake from sticking, line the baking pan with parchment paper before greasing and flouring. The cake can be stored in an airtight container at room temperature for up to 3 days.