Preheat the oven to 350F (180C). Butter and flour a 9-1/2 inch (22.5cm) spring form pan.
Sift together the flour and the salt onto a piece of waxed or parchment paper.
In a small, heavy-bottomed pan melt the butter over medium heat. Place the chocolate in a large, heatproof bowl. When the butter is melted, pour it over the chocolate and whisk until the chocolate has melted into the butter.
In another large bowl, whisk the eggs with the sugar just until thoroughly combined – do not whisk them to a pale yellow. Whisk in the chocolate and the butter mixture, then quickly whisk in the flour and the salt. Pour the mixture into the prepared pan, set the pan on a baking sheet and bake in the center of the oven until the edges of the cake are baked but the center is still soft and tender, 30 to 40 minutes.
Remove the cake from the oven and let cool on a wire rack for 10 minutes before removing it from the mold and transferring it to a