Step 1 Adjust an oven rack to the lower-middle position and heat the oven to 325°F. Line the bottom of an 8-inch springform pan with parchment paper and grease the sides of the pan. Center the pan on a large sheet of heavy-duty aluminum foil and wrap the edges up around the sides of the pan. Repeat with a second sheet of foil. Set the pan in a large roasting pan, or any pan that’s larger than the springform. Bring a kettle or pot of water to boil.
Step 2 In a stand mixer using the whisk attachment, beat the eggs at high speed until the volume doubles. This usually takes about 5 minutes.
Step 3 Meanwhile, melt the chocolate and butter together. You can either do this in a double boiler on the stove (by placing the chocolate and butter in a large heatproof bowl, set over a pan of almost-simmering water, and stirring until melted and smooth). Or you can do this in the microwave (by heating the chocolate and butter in a microwave-safe bowl in 30-second intervals, stirring in between, until the chocolate and butter are melted and smooth). Take care not to overheat the chocolate – you only want to heat it enough so that the chocolate *just* melts.
Step 4 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 egg foam, and then the last half of the foam, until the mixture is totally homogenous. Do not overstir.
Step 5 Scrape the batter into the prepared springform pan and smooth the surface with a rubber spatula. Place the roasting pan on the oven rack and VERY carefully pour in enough boiling water to come about halfway up the sides of the springform pan. Bake for 35-40 minutes, until the sides of the cake begin to pull away from the edges of the pan. Carefully remove the springform pan from the water bath and set it on a wire rack; cool to room temperature. Cover and refrigerate until completely chilled.
Step 6 About 30 minutes prior to serving, run a knife around the edges of the cake and carefully remove the sides of the springform pan. Carefully slip a spatula under the parchment to loosen and remove the pan base and transfer the cake to a serving platter. If desired, lightly dust the cake with powdered sugar and top with berries. To slice, use a sharp, thin-bladed knife, dipping the knife into a pitcher of hot water and wiping the blade before each cut.
