Preheat oven to 350°F/180°C.
Line the pan with parchment paper and grease it with butter.
In a small saucepan, melt the butter with the chocolate over low heat, occasionally whisking until smooth and melted. Let it cool for a few minutes.
Transfer the chocolate-butter mixture to a large bowl, add sugar and salt, and mix well.
One at a time, add the eggs and vigorously whisk to incorporate after each one is added.
Add flour and mix just until everything is combined.
Spread batter into the prepared pan and flatten the mixture with a spatula. The mixture will be very thick, and that is OK.
Bake for 15 to 20 minutes. The cake should be puffed and beginning to crack at the edges but still soft to the touch (a toothpick stuck in the center comes out with moist chocolate crumbs).
Cool to room temperature.