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1
Put oven rack in middle position and preheat oven to 350F.
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2
Butter a 9-inch springform pan and line bottom with a round of parchment or wax paper, then butter paper.
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3
Melt chocolate and butter in a large metal bowl set over a pan of barely simmering water (or in a microwave-safe large glass or ceramic bowl in a microwave at 50 percent power for 4 to 5 minutes), stirring frequently, then cool completely.
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4
Whisk in vanilla, salt, and 6 tablespoons sugar.
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5
Add yolks 1 at a time, whisking well after each addition.
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6
Whisk in flour.
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7
Beat whites with a pinch of salt in a bowl using an electric mixer at medium-high speed until they hold soft peaks, then add remaining 6 tablespoons sugar a little at a time, beating, and continue to beat until whites hold stiff glossy peaks.
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8
Whisk about one fourth of whites into chocolate mixture to lighten, then fold in remaining whites gently but thoroughly.
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9
Pour batter into springform pan, spreading evenly.
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10
Bake until a wooden pick or skewer inserted in center comes out with moist crumbs adhering, 35 to 40 minutes.
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11
Cool cake in pan on a rack 10 minutes.
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12
Remove side of pan and cool cake completely.
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13
Invert cake onto rack and remove bottom of pan, discarding paper, then invert cake onto a plate.
