Preheat the oven to 350 degrees. Line a rimmed 9-by-13-inch baking sheet with parchment paper.
In the microwave, or in the top bowl of a double boiler, melt together the butter and chopped chocolate, stirring until smooth. Meanwhile, combine flour, kosher salt and cayenne in a medium bowl.
Transfer chocolate mixture to a large mixing bowl and whisk in the cocoa powder and sugar. Add the eggs and vanilla; whisk until smooth.
Fold in the dry ingredients and continue folding until no lumps remain.
Scrape batter into the prepared pan and smooth the top with a spatula. Sprinkle all over with Maldon salt. Bake for 25 to 30 minutes until the edges just begin to pull away from the sides of the pan and the top is set and shiny.
Allow the brownies to cool completely in the pan before cutting.