Preheat oven to 325°. Lightly butter and flour pan. Heat chocolate, sugar, and 1 cup butter in a heatproof bowl set over a saucepan of simmering water (bowl should not touch water), stirring, until chocolate is almost completely melted, about 3 minutes; remove from heat and continue to stir until chocolate is completely melted. Let cool completely.
Using an electric mixer on medium speed, add eggs to chocolate mixture 1 at a time, beating to blend after each addition. Beat until mixture has a mousse-like consistency. Reduce speed to low and add salt and ⅓ cup flour; mix until smooth. Scrape batter into prepared pan; smooth top.
Bake cake until top is firm and edges are slightly darkened, 55–65 minutes (rely on visual cues; a tester inserted into cake’s center will come out clean before cake is truly done). Transfer pan to a wire rack and let cake cool completely in pan before turning out.