9 ounces semisweet chocolate, chopped
1 cup unsalted butter
5 large eggs
1/2 cup superfine sugar, plus some for sprinkling
1 tablespoon dark cocoa powder
1/4 teaspoon espresso powder
2 teaspoons vanilla extract
Cocoa powder, for dusting
Chocolate shavings, for decorating.
Preheat the oven to 350° F. Lightly butter a 9-inch spring form pan and line the base with parchment paper. Butter the paper and sprinkle with a little sugar, tapping out excess.
Because the cake is baked in a water bath, carefully wrap the base and halfway up the sides with two thicknesses of heavy-duty aluminum foil to avoid any water leaking into the cake.
Melt the chocolate and butter in a medium saucepan over low heat, stirring frequently. Remove from heat; set aside. In the work bowl of a stand mixer beat the eggs and 1/2 cup of the sugar for one minute.
In a small bowl, mix together cocoa, espresso powder, and the remaining 1 tablespoon sugar; beat into the egg mixture until well blended. Beat in vanilla extract, then slowly beat in the melted chocolate until well blended. Pour mixture into the prepared pan and tap gently on the counter to release any air bubbles.
Place the cake pan into a larger roasting pan, and pour in boiling water to come 3/4 inches up the side of the wrapped pan. Bake for 60 minutes, or until the edge of the cake is set and the center is still a bit soft. The cake is done when a toothpick inserted 2 inches from the edge comes out clean. Lift the pan out of the water, remove the foil, and place on a wire rack. Run a knife around the side of the pan and then remove the side of the springform pan. Let the cake cool completely. Don't be alarmed if it sinks a little in the middle.
Invert the cake onto a wire rack, remove the parchment paper, and dust lightly with cocoa. Sprinkle chocolate shavings around the edge, and then carefully slide the cake onto your serving plate.