Preheat the oven to 350F. Spray 2 8-inch round cake pans with flour-added cooking spray. Line bottoms with rounds of parchment paper.
Combine the flour, sugar, cocoa, baking soda and salt in mixing bowl and stir very well to combine. Add the soft butter and buttermilk and blend on low (I used a stand mixer with a paddle) until smooth.
Whisk the eggs, vanilla and cooled coffee together in another bowl and add to the batter in 3 additions, scraping down the sides of the bowl and mixing until blended. Try not to beat a lot of air into the egg mixture as you add it – just stir well. Scrape bottom of bowl and make sure to mix in all batter.
Divide the batter among the pans and bake on center rack for 35 to 40 minutes or until a wooden toothpick inserted in the center comes out clean. Let cool in pans for about 5 minutes, then turn invert onto wire racks to cool completely. Remove parchment when cool.
Chocolate Frosting: Melt both chocolates in microwave, double boiler or bowl set over barely simmering water; set aside to cool.
Beat the butter until creamy. Make sure it's not too cold or it might cause the chocolate to re-solidify. Stir in the cocoa power, about a cup of the powdered sugar, and the cooled melted chocolate. Scrape sides of bowl and beat well; add remaining powdered sugar, then add room temperature milk, vanilla and salt, beating and scraping until you get the consistency you like.