Ingredients

• 8 ounce dark chocolate, chopped
• 6 ounce unsalted butter
• 5 eggs, separated
• 0.5 cup granulated sugar
• 0.25 cup unsweetened cocoa powder
• 0.25 teaspoon sea salt
• 1 teaspoon vanilla extract
• 4 ounce dark chocolate for ganache, chopped
• 0.33 cup heavy cream
• 1 tablespoon unsalted butter for ganache
• 2 teaspoon light corn syrup
• 1 tablespoon cocoa powder for dusting

Steps

1. Preheat oven to 350°F (175°C). Line a 9-inch round cake pan with parchment paper, allowing edges to overhang slightly for easy removal.
2. Set up a double boiler: place a heatproof bowl over a pot of gently simmering water (bowl should not touch water). Add 8 ounces chopped dark chocolate and 6 ounces butter to the bowl.
3. Stir the chocolate and butter together until completely melted and smooth, about 3 minutes. Remove from heat and let cool for 2 minutes.
4. Whisk the melted chocolate mixture with 1/4 cup cocoa powder, 1/4 teaspoon salt, and 1 teaspoon vanilla extract until well combined. Set aside.
5. Separate 5 eggs, placing yolks in one bowl and whites in another. Add yolks one at a time to the chocolate mixture, whisking after each addition until fully incorporated.
6. Using an electric mixer, beat the 5 egg whites on medium-high speed until soft peaks form, about 2 minutes.
7. Gradually add 1/2 cup granulated sugar to the egg whites while continuing to beat until stiff, glossy peaks form, about 2 more minutes.
8. Gently fold one-third of the egg white mixture into the chocolate mixture using a rubber spatula. Then fold in the remaining egg whites in two additions, being careful not to overmix—a few white streaks are acceptable.
9. Pour half of the cake batter into the prepared pan and spread evenly.
10. Prepare the ganache center: place 4 ounces chopped dark chocolate and 1 tablespoon butter in a heatproof bowl over simmering water. Heat until melted, about 2 minutes. Whisk in 1/3 cup heavy cream and 2 teaspoons corn syrup until smooth. Remove from heat and let cool for 1 minute.
11. Pour the warm ganache center in a thin stream over the batter in the pan, leaving a 1-inch border from the edges.
12. Top with the remaining cake batter, gently spreading it to cover the ganache layer completely.
13. Bake for 20–25 minutes until the edges are set but the center still jiggles slightly when the pan is gently shaken. The cake should register 165°F (74°C) on an instant-read thermometer inserted 2 inches from the edge, but the very center should feel slightly underdone (around 160°F/71°C).
14. Remove the cake from the oven and let it rest in the pan for 5 minutes to set the edges slightly.
15. Invert the warm cake onto a wire rack by placing the rack over the pan and flipping both together. Carefully peel away the parchment paper.
16. Allow the cake to cool to room temperature for about 20 minutes. The ganache center will set slightly but remain soft and fudgy.
17. Dust the top of the cake lightly with cocoa powder using a fine sieve. Slice with a warm, dry knife (wipe between cuts) and serve at room temperature or slightly warm.