01 - Preheat oven to 350°F. Grease and line three 8-inch round cake pans with parchment paper.
02 - Sift together flour, cocoa powder, sugar, baking powder, baking soda, and salt in a large bowl.
03 - Whisk buttermilk, hot coffee, oil, eggs, and vanilla in a separate bowl until well combined.
04 - Add wet ingredients to dry mixture. Mix until just combined; do not overmix.
05 - Divide batter evenly among prepared pans. Smooth tops with a spatula.
06 - Bake for 30–35 minutes, or until a toothpick inserted into center comes out clean.
07 - Cool cakes in pans for 10 minutes, then turn out onto wire racks to cool completely.
08 - Beat butter until creamy. Add powdered sugar and cocoa powder gradually on low speed. Add milk, vanilla, and salt. Beat until fluffy, adding extra milk if needed for spreadability.
09 - Heat cream in a small saucepan until just simmering. Pour over chopped chocolate and butter in a bowl. Let stand 2 minutes, then whisk until smooth. Cool until thickened but still pourable.
10 - Place first cake layer on serving plate. Spread with frosting. Top with second layer, frost again. Add third layer and frost top and sides.
11 - Pour ganache over top, letting it drip down sides for a dramatic effect.
12 - Chill cake for 20–30 minutes to set ganache before slicing.