Preheat the oven to 350°F (175°C). Grease two 9-inch round cake pans and line the bottoms with parchment paper.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
In another bowl, whisk the eggs, buttermilk, vegetable oil, and vanilla until smooth.
Add the wet ingredients to the dry ingredients and mix just until combined.
Slowly pour in the hot brewed coffee while stirring. The batter will be thin
Divide the batter evenly between the prepared pans.
Bake for 28 to 34 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Cool the cakes in their pans for about 10 minutes, then transfer them to a cooling rack. Remove the parchment and cool completely
For the buttercream, beat the softened butter until creamy. Gradually add the powdered sugar while mixing on low speed.
Add the Kahlua, vanilla, espresso powder, and salt. Beat until smooth and fluffy.
Add heavy cream or milk, 1 tablespoon at a time, if the frosting needs to be loosened
Level the cake layers if needed. Place one layer on a serving plate and spread an even layer of buttercream over the top.
Add the second cake layer and cover the cake with a thin crumb coat. Chill for 15 to 20 minutes if the frosting feels soft.
Spread the remaining buttercream over the top and sides of the cake. Decorate with chocolate shavings, cocoa powder, or chocolate-covered coffee beans if desired.
Refrigerate the finished cake for about 20 to 30 minutes before slicing. Let it sit briefly at room temperature before serving.
