Preheat oven to 350°.

Line a 8x8 or 9x9 metal pan with foil, draping the foil over the edges. Lightly butter the foil.

In a large saucepan, melt the butter with the unsweetened chocolate over very low heat, stirring occasionally.

Remove from the heat when melted. Whisking them in one at a time until thoroughly incorporated, add the cocoa, sugar, vanilla, eggs and flour.

Pour the batter in the prepared cake pan and smooth the surface.

Sprinkle with the half teaspoon of sea salt evenly over batter. Using a butter knife, swirl the salt into the batter.

Bake in the center of the oven for about 35 minutes, until the edges are set but the center is still a bit soft and a toothpick inserted into the center comes out with only a trace of the batter.

Let cool at room temperature in the pan for one hour, then refrigerate just until firm, about 1 hour longer.

Lift the brownie from the pan and peel off the foil. Cut into squares, sprinkle with sea salt if desired and serve the brownies at room temperature.