Preheat the oven to 350°F (175°C) and grease a 9x13 inch baking pan.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
Add the buttermilk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Mix until smooth.
Stir in the boiling water until fully incorporated. The batter will be thin.
Pour the batter into the prepared pan and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely before serving.
