Preheat the oven to 350°F / 175℃. Generously grease two 8-inch / 20cm round cake pans 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-rum and whisk until totally blended and smooth. The batter will be quite thin.
Pour the batter into the prepared pans. Gently tap the pans on the counter several times to release excess air bubbles.
Bake until a tester comes out clean, about 30 minutes. Let the cakes cool completely before removing from the pan and inverting onto a serving plate.