1. Preheat the oven to 350°. Place two large pieces of foil in a 9-inch square metal cake pan with foil, draping the foil over the edges. Spray foil with cooking spray.

2. Melt the butter with the unsweetened chocolate over very low heat, stirring occasionally. Remove from the heat. Whisk to combine. Add the cocoa, sugar, eggs, vanilla and flour. Pour the batter into the prepared pan and smooth the surface with a rubber spatula. Grind sea salt evenly over the batter. Using a butter knife, swirl the salt into the batter. Grind a little extra on top, making sure to sprinkle it evenly and leave it setting on top of the batter.

3. Bake for about 35 minutes, until the edge is set but the center is still a bit soft and a toothpick inserted into the center comes out coated with a little of the batter. Cool at room temperature in the pan for about an hour, then refrigerate just until they are firm. Lift the brownies from the pan and remove foil. Cut the brownies into 12 or 16 squares.