Preheat the oven to 350°F (175°C) and grease or line a 9x13-inch baking pan.
In a large bowl, whisk together the flour, sugar, salt, baking powder, and cocoa powder.
In another bowl, whisk together the vegetable oil, buttermilk, eggs, and vanilla extract.
Combine the wet and dry ingredients, stirring gently until blended.
Slowly pour in the hot coffee or water, stirring until the batter is smooth and thin.
Pour the batter into the prepared pan and bake for 25 to 30 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs.
While the cake bakes, melt the butter in a saucepan over low heat.
Whisk in the cocoa powder and milk until smooth.
Remove from heat and gradually add the powdered sugar and vanilla, whisking until glossy and smooth.
Pour the warm glaze evenly over the hot cake.
Allow the cake to cool completely, then cut into bite-sized squares.