Step 1

Place a rack in the bottom third of the oven and preheat the oven to 350 F. Butter a 9-inch springform pan, then line the bottom with parchment paper cut to fit (or a parchment paper round) and butter the parchment paper as well.

Step 2

Fill a saucepan with an inch or so of water, and place a metal or glass bowl in the pot, making sure the bottom is hovering over the water and not touching it. Place the chocolate and the butter in the bowl. Heat the water to almost a simmer over medium heat without allowing it to come to a full simmer. Stir the chocolate and butter until melted and smooth. Remove the bowl from the pan, turn off the heat, and whisk in the sugar. Whisk in the vanilla and salt, then sprinkle the 2/3 cup cocoa powder over the top and beat until smooth. Whisk in one egg at a time, beating until it is completely incorporated before adding the next one so that the eggs blend in thoroughly.

Step 3

Pour the batter into the pan. Bake for 35 to 40 minutes, until the top has formed a delicate crust and a wooden skewer inserted into the center of the cake comes out clean, or almost clean. Cool the cake in the pan on a wire rack for about 20 minutes (it will fall a bit; that’s ok!), then remove the sides of the springform pan. Invert the cake onto a plate, remove the bottom of the pan and the parchment, then turn the cake right side up and finish cooling on the wire rack.

Step 4

When the cake is cool, sprinkle it with a bit of additional cocoa powder and top with berries if you like. Or, frost with Chocolate Ganache and then top with berries.

Step 5

To slice cleanly, run a very sharp, thin knife under very hot water. Wipe off the knife so it’s dry, then cut the cake. Run the knife under hot water and dry it between each slice and you will get super clean slices! Slice and serve with whipped cream and berries if desired.