Preheat oven to 350°F. Grease a 8×8” square baker or 10.5"x7.5" baker.

In a small pot, over low, melt chopped chocolate and butter until smooth.

Add sugar to a large bowl and pour over the melted chocolate. Whisk well while warm until the mixture is glossy.

Whisk in 2 whole eggs, 2 egg yolks, and vanilla until thickened and shiny.

Fold in flour, cocoa, salt, and espresso powder (if using). Batter will be thick.

In a clean bowl, whip 2 egg whites to medium peaks.

Fold ⅓ of the whipped whites into the batter to loosen it, then gently fold in the remaining whites until combined. Avoid overmixing.

Transfer batter to the prepared pan.
Bake 25–32 minutes, until the top is puffed and crackly, edges are set, and the center still jiggles slightly.

Remove from oven and sprinkle with flaky sea salt. Cool 15 minutes before serving. The cake will settle slightly like a soufflé but remain fudgy and gooey inside.

Serve with ice cream.