Preheat oven to 350°F (180°C). Lightly butter a 6-inch springform pan, wiping out excess. (See Note.) Line bottom of pan with parchment paper.

In the top of a double boiler, heat chocolate and butter over simmering water, stirring frequently, until melted and smooth. Remove from heat, and let cool for 10 minutes.

In a medium bowl, beat eggs, granulated sugar, salt, and vanilla with a hand mixer at medium speed until thick and pale, 2 to 3 minutes. Using a balloon whisk, fold in cooled chocolate mixture. Sift cocoa onto mixture, and fold with whisk until completely combined and no dry streaks remain. Gently spread batter into prepared pan.

Bake until a wooden pick inserted in center comes out with a few moist crumbs and an instant-read thermometer inserted in center registers at least 210°F (99°C), 40 to 45 minutes. Let cool in pan for 10 minutes. Carefully remove sides of pan, and let cool completely on pan base on a wire rack.

Just before serving, lightly dust with confectioners’ sugar. Store in an airtight container for up to 3 days.