Preheat your oven to 350°F (180°C). Lightly grease a 6-inch round cake pan and line the bottom with parchment paper.
In a medium bowl, whisk together the sugar, cocoa powder, flour, baking soda, and salt.
Pour in the warm water, oil, vinegar, and vanilla. Stir until just combined—don’t overmix it.
Pour the batter into your prepared pan and smooth out the top.
Bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool completely before removing from the pan. Dust with powdered sugar or top with your favorite frosting if you're feeling fancy.