Preheat the oven to 350 degrees. Line three 8-inch round cake pans with parchment paper. Butter pans and dust with cocoa powder (instead of flour which leaves white splotches on the chocolate layers), shaking off the excess.
In a double boiler, melt dark chocolate. Remove from heat and cool. Quicken this step by cooling chocolate in the refrigerator.
In a medium bowl, sift the cake flour, cocoa powder, baking powder, baking soda and salt. In a small bowl or large measuring cup, whisk together coffee and buttermilk.
In a mixer fitted with the paddle attachment, beat butter and sugar together until fluffy, about three minutes. Add eggs, one at a time, scraping sides of bowl down after each addition. Add vanilla and mix until incorporated. Add the flour mixture in three additions, alternating with the coffee/buttermilk mixture. Begin and end with the flour mixture. Remove the bowl from the mixer and using a spatula, fold in the melted chocolate.
Divide the batter evenly between the three pans. Batter should fill pans about half to two-thirds of the way full. Smooth the tops with a spatula. Bake for 30 to 35 minutes, or until a toothpick inserted in the middle comes out clean. Transfer cakes to a wire cooling rack for 20-30 minutes. Invert cakes onto rack and remove pans and parchment to let cool completely.