In a large bowl, whisk together flour, sugar, cocoa, baking soda, baking powder, and salt until no streaks remain.

Add eggs, buttermilk, oil, and vanilla to the dry ingredients. Whisk from the center outward until a smooth, thick batter forms.

Pour in the hot coffee while whisking slowly. The batter will thin noticeably. Whisk until fully incorporated and uniform.

Divide the batter evenly between the two pans. Tap each pan on the counter twice to release air bubbles. Bake on the center rack for 30 to 35 minutes, until a toothpick comes out with just a few moist crumbs.

Cool in pans on a wire rack for 15 minutes, then turn out, peel away the parchment, and cool completely, at least 45 minutes.

For the buttercream: beat softened butter for 3 minutes until pale. Add sifted cocoa and beat 1 minute more. Add powdered sugar in two additions, beating after each. Add vanilla, salt, and cream one tablespoon at a time, beating until smooth and spreadable, about 4 to 5 minutes total.

Place one cooled layer on a serving plate. Spread buttercream evenly on top. Place the second layer on. Apply a thin crumb coat over the entire cake and refrigerate 20 minutes. Apply remaining frosting in smooth strokes. Serve at room temperature.