Heat oven to 350°F. Line a round baking pan (8 to 9 inches) with parchment paper, butter parchment and sides. Melt chocolate and butter together in a heatproof bowl over simmering water or in microwave. Stir until smooth. Whisk in sugar, then eggs (one at a time), then vanilla and salt. Stir in flour and scrape batter into prepared pan. Bake for 25 to 30 minutes, until toothpick inserted in center comes out batter-free.