FOR THE CAKE

Preheat oven to 350°F (175°C). Butter two 8-inch round cake pans and line the bottoms with parchment paper discs.
Whisk all the dry ingredients together well in a large bowl.
In a separate bowl, whisk together all the wet ingredients EXCEPT the coffee.
Slowly add the wet ingredients to the dry ingredients, mixing as you go. Once incorporated, add the hot coffee, and mix everything until there are no pockets of dry flour left. The batter will be very thin.
Divide the batter between the two cake pans, and bake for 35 minutes until risen and a toothpick inserted into the center of the cake comes out without wet batter clinging to it.
Allow the cakes to partially cool in the pans, then turn out onto a cooling rack to completely cool before frosting. The cakes can be made a day or two ahead and kept in the refrigerator before frosting.
FOR THE FROSTING

Melt the chocolate in a double boiler or in the microwave using short bursts. Stir until smooth and set aside to cool slightly.
Beat the room temperature butter in the bowl of a stand mixer with the paddle attachment for a couple of minutes until smooth and noticeably paler in color. Slowly add the powdered sugar, salt, and vanilla, beating on a low-medium speed until everything is smooth.
Whisk the sour cream into the partially cooled melted chocolate. This both cools down the chocolate and warms up the sour cream so that both are more-or-less room temperature before adding to the butter mixture.
Add the chocolate and sour cream mixture to the mixer bowl and beat on low speed until everything is smooth and combined.
If your frosting seems a little too loose, you can refrigerate it for 10-15 minutes to allow it to firm up a little bit. Then give it a good stir before frosting the cake.