Preheat the oven to 400 degrees F (200 degrees C). Generously butter a 9x2 inch round cake pan and dust with flour.
Melt butter and chocolate together in a heavy saucepan over very low heat. Remove from heat and whisk in sugar and eggs 1 at a time. Stir vigorously 2 or 3 times, then gently whisk in flour and salt. Pour batter into the prepared pan. Rap pan once against the counter to settle batter.
Bake in the preheated oven until a knife inserted in the center of the cake comes out clean, about 20 minutes.
Run a knife around the edge of the pan and cool for 10 minutes. Unmold cake and cool right-side up on a wire rack.
