Step 1

Preheat the oven to 325 degrees F (165 degrees C). Grease and flour a 10x15-inch baking pan.

Step 2

Combine flour, sugar, oil, buttermilk, cocoa, eggs, baking soda, vanilla extract, and salt in a large bowl; beat with an electric mixer until smooth and creamy. Add boiling water and mix well. Pour into the prepared baking pan.

Step 3

Bake in the preheated oven for 15 minutes. Increase oven temperature to 350 degrees F (175 degrees C) and continue to bake until a toothpick inserted into the center comes out clean, 20 to 25 more minutes.