Heat oven to 350°F (175°C). Grease two 8-inch round pans and line with parchment. Whisk flour, sugar, cocoa, baking powder, baking soda, and salt in a large bowl until combined.

Add buttermilk, oil, eggs, and vanilla. Whisk until just combined. Stir in hot coffee until the batter is smooth and thin.

Divide batter between pans. Bake 28 to 32 minutes, until tops spring back. Cool in pans 10 minutes, then turn out onto a rack to cool completely.

For the frosting, beat butter until creamy. Add powdered sugar, cocoa, 3 tbsp milk, vanilla, and salt. Beat until fluffy, adding more milk if needed.

Place one layer flat side up on a plate, spread with a third of the frosting. Add second layer flat side down, frost top and sides. Slice and serve.