Position a rack in the lower third of the oven and preheat to 325 degrees . Butter a 9-inch springform pan and line the bottom with parchment, then butter the parchment; add 1/4 cup almond flour and tilt the pan to coat all sides. In a double boiler, melt 1 1/2 sticks (6 ounces) butter and 9 ounces chopped bittersweet chocolate over simmering water, stirring; remove from the heat. Whisk in 1/2 cup sugar and let cool slightly. Using an electric mixer, beat 6 egg whites and 2 pinches salt at medium speed until foamy, about 2 minutes; sprinkle in another 1/2 cup sugar. Increase the speed to high and beat until moist, stiff peaks form. Whisk 6 egg yolks and 1 1/4 cups almond flour into the cooled chocolate mixture. Fold in one-quarter of the egg white mixture until nearly blended; fold back into the remaining egg white mixture. Scrape into the prepared pan. Transfer to the oven and place a baking dish filled halfway with boiling water on the oven floor; bake for 1 hour. Let cool for 30 minutes before removing the sides of the pan.