Preheat the oven to 350 degrees F. Coat the bottom and sides of two 9-by-2-inch round cake pans with cooking spray and line the bottoms with parchment paper.
Whisk the cocoa powder and hot water in a small bowl until smooth; let cool. Whisk the flour, salt and baking soda in a medium bowl. Beat the oil, sugar, buttermilk, eggs, food coloring, vanilla and vinegar in a large bowl. Whisk in the cocoa mixture until smooth, then whisk in the flour mixture. Divide the batter between the pans.
Bake the cakes until the centers spring back when lightly pressed and a toothpick inserted into the centers comes out clean, 30 to 40 minutes. Transfer to a rack and let cool 15 minutes in the pans. Loosen the edges with a knife and turn out the cakes onto the rack; remove the parchment and let cool completely.