Use parchment paper to line an 8x8 baking dish. This helps to make it easier to remove the cake.
In a mixing bowl, whisk together the dry ingredients. Mix the flour, sugar, cocoa powder, baking soda, and salt together thoroughly.
Once the dry ingredients are mixed, make a well in the center.
Pour in the vinegar, vanilla extract, vegetable oil, and warm water. Mix until just combined but don't overmix it.
Transfer the batter to the lined baking dish.
Bake in the oven for about 30 minutes. To test, insert a toothpick. When it comes out with moist crumbs, it's perfect. Set it aside to cool.