Preheat the oven to 350°F. Lightly grease 7" or 8" round cake pans with cooking spray. Line the the bottom with parchment paper, then grease the parchment as well.
In a large mixing bowl, sift the flour, sugar, cocoa, baking soda and salt. Mix well.
Combine water and milk in a measuring cup or a small bowl. Warm them until they are quite hot to touch, about 1 minute in the microwave. Add oil, vinegar, vanilla extract to the hot milk mixture and mix well until combined.
Add the wet ingredients into the bowl with dry ingredients. Whisk until well combined and there are no pockets of flour anywhere. DO NOT overmix.
Pour the into the prepared baking pan and bake in the oven for 35~40 minutes or until a toothpick inserted into the center of the cake comes out clean. Remove from the oven and let cool in the pan for 5~10 minutes.
Then carefully flip the pan over a wire rack. Peel off the parchment and let the cake cool completely on the wire rack.