Preheat oven to 300 degrees F and grease pans. Line bottoms with rounds of wax paper or parchment paper and grease paper.
Finely chop chocolate and combine with hot coffee in a bowl. Let mixture stand, stirring occasionally, until chocolate is melted and mixture is smooth.
In a large bowl sift together sugar, flour, cocoa powder, baking soda, baking powder, and salt.
In another 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 coffee mixture to eggs, beating until combined well. Add sugar mixture on medium speed until just combined well. Batter will be very thin.
Divide batter between pans and bake in middle of oven until tested inserted in center comes out clean, 1 hour to 1 hour and 10 minutes.
Cool layers completely in pans on racks. Run a thin knife around edges of pan and invert layers onto racks. Carefully remove wax paper and cool layers completely. Cake layers may be made 1 day ahead and kept, wrapped well in plastic wrap, at room temperature.