Preheat oven to 350°F (180°C)

Melt chocolate and butter in a double boiler, stirring occasionally, until smooth

Whisk in sugar until well combined

Beat in eggs one at a time, followed by vanilla extract

Sift in flour and salt, then fold into the chocolate mixture until just combined

Stir in sea salt

Pour batter into a lined or greased 8-inch square baking pan

Bake for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs attached

Remove from the oven and let cool completely in the pan

Cut into squares and serve