Preheat your oven to 350 degrees. Spray a loaf pan with cooking spray, and line with parchment paper with overhang to help remove the cake after baking. Set aside.
In a medium bowl, stir together the flour, sugar, cocoa powder, baking soda, and salt. In a small bowl, stir together the water, oil, vanilla extract, and vinegar. Pour the wet ingredients into the dry, and whisk together until smooth and combined.
Pour the batter into your prepared loaf pan and spread evenly. Bake for 18-20 minutes, or until a toothpick comes out clean. Cool completely before removing from the pan and frosting.
In a medium bowl, beat together the butter and sour cream to combine. Add in the powdered sugar, cocoa powder, vanilla, and salt. Beat for several minutes, until completely combined and smooth. It should be thick, but still creamy and spreadable. Add more powdered sugar 1 tablespoon at a time if it's too thin. Spread on the cooled cake generously, and slice to serve!