Preheat oven to 350°F and grease a half sheet pan (18x13).
In a large mixing bowl, whisk together the all purpose flour, cocoa powder, baking soda, and salt.
In a stand mixer or with hand beaters, beat together the canola oil, eggs, sugar and vanilla extract until smooth.
Add a third of the dry ingredients and mix just until combined.
Next, add half of the buttermilk and mix just until combined and scraping the sides of the bowl as needed.
Add another third of the dry ingredients followed by the remaining buttermilk mixing just until combined.
Now add the remaining dry ingredients just until combined.
Add the hot water and mix until everything is fully incorporated and smooth.
Pour cake batter into prepared pan and bake for 20-25 minutes until a toothpick inserted in the center comes out clean with just a few crumbs.
Let cake cool completely before frosting.