To begin, combine the strong coffee and bittersweet chocolate in a saucepan over low heat. Stir these around until the chocolate is melted, then remove it from the heat and set aside for now.
Preheat your oven to 350 degrees.
In a bowl, sift together the flour, baking soda, baking powder, and salt. Set that aside for now.
In your mixing bowl, beat the butter until it is creamy. Add in the granulated sugar and beat those together until they are fluffy. Add in the eggs, one at a time, mixing each in completely before adding the next.
Mix half of your flour mixture into your creamy mixture. When it is combined, add in half of your coffee & chocolate liquid and mix it in.
Repeat the process with the other half of the dry ingredients, followed by the remaining chocolate & coffee liquid. Stir in the vanilla extract.
Divide the mixture into two 8-inch prepared baking pans. Butter and flour them if using traditional pans. I use spring-form pans, so I lined the bottoms with parchment paper, then sprayed the bottom & sides with baking spray.
Bake the cakes layers for about 30 minutes, or until they bounce back in the center when lightly touched and the edges have begun to shrink away from the sides.
Turn the cakes out onto cake racks and cool completely.
Now for the Icing:
In a small dish, combine the warm milk and the instant espresso powder. Stir until the powder is dissolved and set it aside.
In your mixing bowl, beat together the butter with the powdered sugar until smooth.
Mix in the baking cocoa, then add in the espresso & milk mixture.
Finally, mix in the dark rum.
Beat the frosting until smooth & creamy.
Spread a layer of frosting on 1 cooled cake, then place the second cake layer on top. Spread the frosting all over.
I honestly don’t worry too much about a perfectly frosted cake. I mean, it isn’t going to look pretty for long, right?