Preheat oven to 350°F (175°C) with rack in center. Grease two 8-inch round cake pans, line bottoms with parchment, grease parchment, and dust with black cocoa or flour.
In a large bowl, whisk together flour, black cocoa, sugar, baking powder, baking soda, and salt until evenly combined.
In a separate bowl, whisk buttermilk, oil, eggs, vinegar, and vanilla until smooth and fully combined.
Pour wet ingredients into dry ingredients and mix on low until just combined, about 1 minute. Slowly pour in hot coffee and mix until incorporated; batter will be thin.
Divide batter evenly between prepared pans. Tap pans on counter to release air bubbles. Bake for 30 to 35 minutes until a toothpick comes out clean.
Cool cakes in pans on a wire rack for 15 minutes. Run a knife around edges, invert onto rack, remove parchment, and cool completely, about 1 hour.
In a stand mixer with paddle attachment, beat cream cheese and butter on medium-high until light and fluffy, 3 to 4 minutes. Gradually add sifted powdered sugar, then vanilla and salt. Beat until silky smooth.
Place one cooled cake layer on a plate, flat side down. Spread frosting over it, top with second layer. Apply a thin crumb coat, chill 15 minutes, then add final layer of frosting. Decorate as desired.