Preheat the oven to 350 degrees F. Butter a 9-inch springform pan and line the bottom with parchment and then butter the parchment.

In a medium saucepan, bring 1 inch of water to a low simmer. Place the chocolate and butter in a large heatproof bowl over the saucepan without the water touching the bottom of the bowl. Stir frequently until the mixture is fully melted and then remove from the heat. Stir in the vanilla extract, 1//2 teaspoon salt and 6 tablespoons sugar. Stir in the yolks one at a time, mixing after each addition. Fold in the flour.

Using an electric mixer, beat the egg whites with the remaining salt until soft peaks form. Continue beating and gradually add the remaining sugar, and mix until the whites hold stiff peaks.

Fold the egg whites into the chocolate mixture and spread the batter into the prepared pan. Bake the cake until it is just set, about 22-25 minutes.

Cool the cake in the pan for 10 minutes before removing the sides. The cake will fall as it continues to cool. Generously dust with confectioners’ sugar before serving.

The cake is very delicate, so it is best to slice and serve it off the pan.