Step 1

Arrange a rack in the middle of the oven and heat the oven to 350°F. Grease a 9-inch springform pan with butter.

Step 2

Place 1 1/2 sticks unsalted butter and 12 ounces finely chopped dark chocolate in a large microwave-safe bowl. Microwave in 30-second intervals until the chocolate is mostly melted, stirring with a spatula between each interval, 2 1/2 to 3 minutes total. Set aside to cool for 5 minutes.

Step 3

Add 2 large eggs, 4 large egg yolks, 1 teaspoon vanilla extract, and 1/4 cup of the granulated sugar. Whisk to combine.

Step 4

Place 4 large egg whites in a large bowl and use an electric hand mixer to beat on medium speed until foamy. With the mixer running, gradually beat in the remaining 1 cup granulated sugar. Continue to beat on medium-high speed until soft peaks form, about 8 minutes. (Alternatively, use a stand mixer, which will take 5 to 6 minutes.)

Step 5

Stir 1/3 of the whipped whites into the chocolate mixture. Then gently fold the remaining egg whites into the chocolate mixture with a large spatula — swoop the spatula through the bottom of the bowl using the spatula to drag the chocolate mixture into the egg whites. Pour the batter into the prepared pan.

Step 6

Bake until the cake is doubled in size, cracked, and the center is set, 35 to 40 minutes. Let the cake cool for 10 to 15 minutes before releasing the sides of the pan. (As the cake cools, the top will shrink and crack.) Let cool completely before serving. Top with whipped cream and berries before serving, if desired.