Preheat the oven to 290 degrees. Line a 9-inch springform pan with parchment paper and wrap the outside twice with foil.
Melt the chocolate and butter in a microwave-safe bowl at 30-second intervals, stirring between each until smooth. Set aside to cool.
In a mixer, whisk the eggs and sugar on high for 5 to 7 minutes until pale and thick.
Slowly add the melted chocolate and cocoa powder, mixing with a rubber spatula until well combined.
Add the salt and mix again until well combined.
Pour the batter into the prepared pan. Then, place the pan in a larger baking dish. Add hot water to the outside dish until it reaches about ¼ of the way up the side of the inner pan.
Bake the cake for 50 to 60 minutes until the top forms a thin crust, but the center is still soft.
Remove the cake from the water bath and cool to room temperature.
Chill the cake for 4 hours and enjoy.
Notes
Storage
Refrigerate this cake for up to 4 days in a covered container.
For longer storage, wrap individual pieces in plastic wrap and freeze them for up to a month in an airtight, freezer-safe container.
Defrost the cake overnight in the refrigerator before serving. Always keep the cake cold.
General Tips
When wrapping the pan with foil, ensure that the seams on the first and second layers are staggered to help make it even harder for water to get into the pan.
We find that the thickness of heavy-duty foil is perfect for wrapping pans for water baths.
If you choose to add berries, wash and dry them just before you add them to the top of the cake to prevent mold growth.