Preheat the oven to 350 degrees F. Generously grease a 9 by 5-inch loaf pan then line with greased parchment paper. Set aside.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt until well combined. In a small bowl, stir together the coffee and rum.
In a separate bowl, whisk together the eggs, buttermilk, oil, and vanilla until well combined. Add the egg mixture and half the coffee rum to the dry ingredients and mix with a spoon until smooth. Slowly add the remaining coffee mixture and whisk until completely blended and smooth. The batter will be quite thin.
Pour the batter into the prepared pan. Gently tap the pan on the counter several times to release excess air bubbles.
Bake until a tester comes out clean, about 1 hour 10 minutes (30 minutes for 8-inch rounds). Let the cake cool completely before removing from the pan and inverting it onto a serving plate. (If you want to trim the top of the cake – which will be the base – do it now; see above.)
Using a metal offset spatula, cover the cake with an even layer of frosting. Top with chocolate shavings, if you’d like. Serve at room temperature.