1/2 cup water
1/2 cup sugar
1 stick (4 ounces) unsalted butter, cut into 10 or 12 pieces
12 ounces bittersweet chocolate (60 to 65 percent), cut into 1/4-inch pieces
6 large eggs
1 tablespoon orange, finely grated zest-only
2 tablespoons dark rum
Center a rack in the middle of the oven and preheat to 325 degrees. Lightly grease a 2-inch-deep, 8-inch-round baking pan; line the bottom with a disk of buttered parchment. Set the pan in a small roasting pan. Boil a kettle of water and set aside.
Combine the 1/2 cup water and sugar in a medium saucepan and place over medium heat. Bring to a boil, stirring occasionally. Add the butter in increments and continue cooking until it is melted and combined. Remove from the heat and add the chocolate, shaking the pan to make sure all the chocolate is submerged. Let stand for 5 minutes, then whisk until smooth.
In a medium bowl, whisk together the eggs, orange zest and rum until well combined. Add the chocolate mixture, whisking it into the eggs in a steady stream, just until the batter is smooth. Transfer the batter to the round pan, then pull out the center oven rack halfway and place the roasting pan on it. Pour the boiled water into the roasting pan so that it comes about 3/4 inch up the side of the round pan. Bake for 35 to 45 minutes, removing it from the oven as soon as the cake has risen slightly and become firm to the touch. (If it is still soft and jiggles in the center, bake for up to 15 minutes, checking frequently.)
Transfer the cake, in the pan, to a wire rack; cool to room temperature. Invert onto a serving platter, removing the pan and the parchment paper bottom.
Cover with plastic wrap and hold at room temperature until ready to serve. Slice the cake with a thin, sharp knife dipped into hot water and wiped after every cut.
This is the perfect emergency recipe. It takes about 10 minutes to assemble and doesn’t call for unusual ingredients.
You can increase the recipe by 1 1/2 times and bake the cake in a 10-inch round pan.
Serve with a little sweetened whipped cream.
For best results, make the cake up to 1 day in advance; cover and store at room temperature (do not refrigerate).