Begin by preheating your oven to 350°F (175°C) and preparing your baking vessel. Butter a 9-inch springform pan and dust it thoroughly with cocoa powder, tapping out any excess. If you're concerned about water leakage from your springform pan, wrap the outside completely with aluminum foil, guaranteeing it extends all the way up to the outer rim.
Next, melt 10 ounces of coarsely chopped bittersweet chocolate and 7 ounces of butter together using a double boiler or microwave, stirring occasionally until the mixture is completely smooth. Once melted, remove the chocolate and butter from heat.
In a large bowl, whisk together 5 large eggs at room temperature with 1 cup of sugar until well combined. Gradually whisk the melted chocolate mixture into the egg and sugar mixture until the batter is smooth and fully incorporated.
Pour the batter into your prepared springform pan and cover the top snugly with a sheet of foil to prevent any water from the water bath from entering the cake.
Place the springform pan into a larger roasting pan or baking pan, then add enough hot water to the outer pan to come approximately halfway up the sides of the cake pan. This water bath guarantees even, gentle baking that prevents cracking and creates the cake's signature dense, fudgy texture.
Bake for 1 hour and 15 minutes. The cake will continue to set as it cools, so avoid opening the oven door during baking.
