Preheat oven to 350 degrees. Prepare 9 inch springform pan (or a 9 inch cake pan lined completely with parchment paper) Butter pan and dust with cocoa powder. Set aside. In a heavy medium saucepan, melt butter on medium low heat, add baking chocolate pieces and carefully stir until melted. In a medium mixing bowl, whisk together large eggs, add granulated sugar, espresso powder and cocoa powder until fully combined. Carefully incorporate melted chocolate and butter until th batter is blended well. Pour batter into prepared pan lightly smooth batter. Bake at 350 degrees on center rack until puffed and set, approximately 35 minutes. Allow cake to cool for a full hour in the pan. If needed run a knife around sides of pan (if a baking pan is used, lift out of pan and fold down parchment ) unmold cake on serving platter. (Cake may be wrapped well after cooling and stored for up to 2 days before serving.) When ready to serve, dust with powdered sugar, add whipped cream and garnish as desired. This is a very rich dense cake, small slices are preferable. 12-16 slices.