Preheat oven to 350°F. Line an 8x8-inch glass or ceramic baking dish with parchment paper and spray sides and bottom. If using a metal baking pan, heat oven to 325°F.
Mix flour, sugar, cocoa powder, baking powder, baking soda, and salt in the bowl of stand mixer.
Add in egg, milk, oil, and vanilla and mix on low until incorporated. When mixed in increase speed to medium and mix for 2 minutes.
Add in warm water and gently stir- batter will be very thin.
Pour batter into prepared dish. Bake for 35-40 minutes, or until an inserted toothpick is removed with a few crumbs. There should not be wet batter, but a few crumbs is good.
Cool for 10 minutes in the pan, then continue cooling on a rack until room temperature.