Preheat your oven to 350°F (180°C). Grease and flour the 9x13 inch baking pan.
In a large mixing bowl, combine the flour, cocoa powder, sugar, baking powder, baking soda, and salt. Whisk them together until well mixed.
In another bowl, mix together the buttermilk, vegetable oil, eggs, and vanilla extract. Stir until fully combined.
Gradually add the wet ingredients to the dry ingredients, mixing on low speed until just combined. Be careful not to overmix.
Slowly stir in the boiling water to the batter until smooth. The batter will be thin, which is normal.
Pour the batter into the prepared baking pan and bake in the preheated oven for 30 minutes or until a toothpick inserted in the center comes out clean.
Once baked, allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.
For an extra touch, you can top the cake with chocolate ganache or your favorite frosting once it has cooled.