Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking soda, and salt. Whisk together until well mixed.
Add the vegetable oil, buttermilk, eggs, and vanilla extract to the dry ingredients. Mix until just combined.
Carefully stir in the boiling water until the batter is smooth. The batter will be thin, which is normal.
If using, fold in the chopped nuts and chocolate chips.
Pour the batter into the prepared baking 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 about 10 minutes before transferring it to a wire rack to cool completely.