8 large eggs, cold
1 lb. bittersweet or semisweet chocolate, coarsely chopped
16 tbsp. unsalted butter, cut into 16 pieces
Preheat the oven to 325° F and adjust an oven rack to the lower-middle position. Grease the sides of an 8-inch springform pan* and line the bottom with parchment paper. Wrap the outside of the pan tightly with heavy duty foil and set in a larger baking pan. Bring a pot of water to boil.
In the bowl of a stand mixer fitted with the whisk attachment beat the eggs at high speed until the volume doubles to approximately 1 quart (about 5-10 minutes).
Meanwhile, melt the chocolate and butter in a heatproof bowl set over a pan of simmering water. Heat until smooth and very warm (about 115° F), stirring once or twice. Fold about a third of the beaten eggs into the chocolate mixture using a large rubber spatula until only a few streaks of egg are visible. Fold in half of the remaining eggs, and then the last half, until the mixture is homogenous.
Scrape the batter into the prepared springform pan and smooth the surface with a spatula. Pour enough boiling water into the larger pan surrounding the springform to come about halfway up the sides of the springform. Bake until the cake has risen slightly, the edges are just beginning to set, and an instant-read thermometer inserted halfway into the center reads 140° F, 22-25 minutes. Remove the springform from the water bath and set on a wire rack; cool to room temperature. Cover and refrigerate overnight to mellow the flavor. The cake can be refrigerated for up to 4 days.
About 3o minutes prior to serving, carefully remove the sides of the springform pan, invert the cake onto a plate to peel off the parchment paper, and reinvert onto a serving platter. Sprinkle lightly with confectioners’ sugar. Use a sharp, thin knife to slice the cake, dipping it into a pitcher of hot water and wiping the blade before each cut.
*Note – a 9-inch springform may be used as well, but you will need to adjust the baking time to 18-20 minutes.