1Preheat oven to 350 degrees F. Line a 9" pan with aluminum foil; grease and flour foil.
2In large saucepan over low heat, melt butter, sugar and coffee together, whisking to blend. Add bittersweet (or semisweet) and unsweetened chocolates. Whisk constantly until melted.
3Remove mixture from heat and whisk in eggs gradually. Pour batter into prepared pan.
4Bake for 50-55 minutes. Cake is done when top has a c rust and is dry to the touch. Cool to room temperature, then refrigerate 12-14 hours before serving. Dust with powdered sugar or top with ganache.
5Dust with powdered sugar or top with ganache. Another alternative is to split the layer and fill with ricotta cream for a real Italian flair.