Preheat oven to 325°F. Lightly butter 9-inch springform pan.

Place butter and chocolate in medium saucepan and melt them together on medium-low heat. Stir in vanilla and set aside.

Using a stand mixer fitted with whisk attachment, whip egg whites on medium-low speed until foamy, about 1 minute. Increase speed to medium-high and continue to whip, slowly adding 1/2 cup granulated sugar, until whites are glossy and thick and hold stiff peaks, about 4 minutes longer. Transfer whites to large bowl.

Add egg yolks to the mixing bowl (no need to wash the bowl — just scrape out as much of the eggs whites as possible) and remaining 1/2 cup granulated sugar and whip on medium-high speed until thick and pale yellow, about 3 minutes, scraping down sides as necessary. Add chocolate mixture and mix on medium speed until incorporated, about 15 seconds. Add almond flour, cocoa, and salt and mix until incorporated, about 30 seconds.

Remove bowl from mixer and stir few times with large rubber spatula, scraping bottom of bowl to ensure almond flour is fully incorporated. Add one-third of whipped whites to bowl, return bowl to mixer, and mix on medium speed until no streaks of white remain, about 30 seconds, scraping down bowl halfway through mixing. Transfer batter to bowl with remaining whites. Using large rubber spatula, gently fold whites into batter until no streaks of white remain. Spread batter into prepared pan, smooth top with spatula, and place pan on rimmed baking sheet.

Bake until toothpick inserted in center comes out with few moist crumbs attached, about 50 minutes. Let cake cool in pan on wire rack for 20 minutes. Remove side of pan and let cake cool completely, about 1 hour. (Cake can be wrapped in plastic wrap and stored at room temperature for up to 3 days.)

Dust top of cake with confectioners’ sugar, if using. Using offset spatula, transfer cake to serving platter. Cut into wedges and serve with a dollop of whipped cream (optional).