Heat oven to 350°F (175°C). Butter two 9-inch round pans and line with parchment. In a heatproof bowl, whisk the cocoa with the hot coffee until smooth. Let it bloom for 5 minutes.

Whisk the flour, sugar, baking soda, baking powder, and salt in a large bowl until combined.

Pour the cooled cocoa mixture into the dry ingredients. Add the buttermilk, oil, eggs, and vanilla. Beat on low just until smooth, about 30 seconds. Scrape the bowl once.

Divide the batter between the pans and bake 30 to 34 minutes, until a toothpick has moist crumbs. Cool in pans 10 minutes, then turn out onto a rack and cool completely.

Beat the butter until fluffy, then mix in the cocoa. On low, add the powdered sugar and cream in additions, then the vanilla and salt. Whip on medium for 2 minutes, adding a little cream if too stiff.

Stack the layers with frosting between, then frost the top and sides. Slice with a hot dry knife and serve at room temperature.