6 1/2 Tablespoons Salted Butter, plus more for the pan
Preheat the oven to 325 degrees F. Grease a 9-inch cake pan with butter and line the bottom with a round of parchment paper.
Pour an inch or two of water into a medium saucepan and bring to a simmer. Combine the butter and chocolate in a heatproof bowl and set on top of the pan of simmering water. Stir until the butter and chocolate are melted. Remove from the heat and let cool for 10 minutes.
In a medium bowl, whisk together the eggs and sugar until light and fluffy. In another bowl, sift the flour and add the fleur de sel.
Stir the melted cho colate mixture into the egg mixture. Add the vanilla. Fold in the flour until just combined. Pour the batter into the prepared cake pan and bake for about 30 minutes.
Let cool in the pan for at least 1 hour before unmolding. Serve topped with berries, if desired, and a sprinkling of confectioners' sugar.