Melt the dark chocolate and butter in a double boiler.In a bowl, beat the eggs with the sugar until the mixture is frothy and white.Pour the chocolate-butter mixture over the eggs-sugar, mixing well. Then add the sifted flour and mix until the dough is smooth.
Butter your cake pan and lightly flour the inside of the pan. Pour the batter into the pan and sprinkle a little fleur de sel on the top surface.
Bake for 25 minutes (or more depending on the size of your pan).Remove from oven and let cool before serving. Serve when the fondant is at room temperature.Enjoy!