Heat oven to 350°F. Grease and line two 9-inch round cake pans.
Whisk flour, sugar, cocoa, baking powder, baking soda, and salt in a large bowl.
In another bowl, whisk buttermilk, oil, vanilla, and vinegar until smooth.
Pour wet mixture into dry ingredients. Stir just until no dry spots remain.
Stream in hot coffee and whisk until the batter is thin and even.
Divide batter between pans. Tap pans gently to release bubbles.
Bake until a toothpick comes out clean, about 28–32 minutes.
Cool in pans 10 minutes. Turn out onto racks and cool completely.
Beat butter for frosting until light and fluffy, about 2 minutes.
Mix in cocoa and half the powdered sugar on low until combined.
Add cream, vanilla, salt, and remaining sugar. Whip until smooth and spreadable.
Frost cooled cakes. Stack, crumb-coat, chill briefly, then finish frosting.