11 tablespoons (1 stick plus 3 tablespoons) unsalted butter
1/2 teaspoon vanilla extract
4 eggs, at room temperature
1/2 cup granulated sugar
1 cup heavy cream, softly whipped with 2 tablespoons confectioners' sugar
1. Set the oven at 350 degrees. Butter a 9-inch springform pan with removable sides. Line the bottom with a round of parchment paper cut to fit it exactly. Butter the parchment. Line the outside of the pan with a double-thickness of foil, pressing the foil onto the sides. (This prevents water from seeping in through the bottom of the pan during baking.)
2. In a heatproof bowl set over a saucepan of hot, but not boiling, water, combine the chocolate and butter. Heat, stirring constantly, just until the chocolate melts. Remove the bowl from the pan and wipe the bottom with a clean cloth. Stir in the vanilla and set it aside to cool slightly.
3. In an electric mixer, beat the eggs and sugar at high speed for 8 minutes or until thick and pale yellow.
4. Fold one-third of the egg mixture into the melted chocolate to lighten it. Then fold the remaining egg mixture into the chocolate as lightly as possible until no yellow patches show.
5. Pour the batter into the prepared pan and set the pan in the center of a large roasting pan. Pour enough hot tap water into the roasting pan to come about one-third of the way up the side of the cake pan. Carefully place the roasting pan in the middle of the oven. Bake 43 to 45 minutes or until the top of the cake is set with a firm crust, but the inside is still moist.
6. Carefully remove the roasting pan from the oven. Lift the pan out of the water, run a thin knife around the edge of the cake, then set the cake in the pan on a rack to cool completely.
7. Remove the foil from the pan (don't release the sides). Cover the pan with a clean piece of foil and chill for at least 4 hours or overnight.
8. One to two hours before serving, remove the cake from the refrigerator. Release the spring on the pan and remove the ring. Invert the cake onto a flat plate or cookie sheet. Insert a narrow metal spatula knife between the bottom of the pan and the parchment paper, moving it just enough to release the bottom of the pan. Peel off the parchment. Invert the cake onto a serving plate and let it sit at room temperature until serving. Cut the cake into thin slices and serve with a dollop of whipped cream. Lisa Zwirn
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