Heat the oven to 350°F.
In an 8x8-inch square baking pan, whisk together the 1 1/2 cups all-purpose flour, 1 cup sugar, 3 tablespoons unsweetened cocoa powder, 1 teaspoon baking soda, and 1/2 teaspoon salt.
Make a well in center of flour mixture and add the 1 cup hot coffee (or hot water, if using), 6 tablespoons vegetable oil, 1 tablespoon vinegar, and 1 teaspoon pure vanilla extract. Whisk until well combined. Make sure to get into all the corners for any pockets of dry ingredients.
Bake until a toothpick inserted in center comes out clean, 35 to 40 minutes. Let cool completely in pan on a wire rack.
Once cool, spread chocolate frosting over the top. Cut cake into 6-8 slices and serve. You can also skip the frosting and sprinkle the top of the cake with powdered sugar.