Direction :

1. Make the cake: Preheat oven to 300°F. and grease two 9- or 10-inch by 2-inch round cake pans with unsalted butter or nonstick spray. Line bottoms with rounds of parchment paper. Finely chop chocolate and in a bowl combine with hot coffee. Let mixture stand, stirring occasionally, until chocolate is melted and mixture is smooth.
2. Into a large bowl sift together sugar, flour, cocoa powder, baking soda, baking powder, and salt. In another large bowl with an electric mixer, beat eggs until thickened slightly and lemon colored (about 3 minutes with a standing mixer or 5 minutes with a hand-held mixer). Slowly add oil, buttermilk, vanilla, and melted chocolate mixture to eggs, beating until combined well. Add sugar mixture and beat on medium speed until just combined. Divide batter between pans (being sure not to fill pans higher than 2/3 full—they will overflow if you do; it’s not worth it) and bake in the middle of oven until a tester inserted in center comes out clean, 1 hour to 1 hour and 10 minutes.
3. Meanwhile, make the icing: Because several commenters have had trouble, with the icing, I am suggesting you use a double boiler. If you know how to set up a double boiler, do so; otherwise, fill a pot with an inch of water. Place a medium bowl over top. Turn the heat to medium. Place the chocolate in the bowl. Once it has melted, stir in the confectioners’ sugar, whisking until smooth. Stir in the eggs a little at a time, whisking vigorously after each addition — a flat-bottomed whisk is good for this. (I use a whisk until the ingredients are blended and then switch to a heat-proof spatula.) Cook over low to medium-low until thick. Stir in the butter one tablespoon at a time. Set aside to cool. Transfer to a glass measuring cup with a spout for easy pouring or store in whatever vessel you wish.
4. Cool layers completely in pans on racks. Run a thin knife around edges of pans and invert layers onto racks. Remove parchment paper. Cake layers may be made 1 day ahead and kept, wrapped well in plastic wrap, at room temperature.
5. If you are making a layer cake, halve each cake round crosswise to create 4 layers. Spread icing over one cake layer. Top layer with other cake layer. Repeat until all layers are stacked. Pour icing on top of top layer and with an off-set spatula (if you have one) spread it all over the sides. Cake keeps, covered and chilled, 3 days. Bring cake to room temperature before serving. (I keep mine at room temperature always…there’s never space in the fridge.)