Preheat oven to 350°F. Line a large baking pan with parchment paper.
Melt together chocolate and butter in the microwave in short bursts with frequent stirring until completely melted, smooth and glossy.
Beat egg white with sugar in a medium bowl for 3-4 minutes until pale and thick. Mix vanilla and salt. Mix in warm chocolate mixture until evenly blended. Mix in baking powder. Sprinkle in flour and fold it in until well incorporated and smooth. Cover and refrigerate the batter for 30 minutes.
Spoon level tablespoons of chilled batter onto prepared baking tray. Make sure you get 10 cookies because if there is too much batter per cookie, they will spread too much. Roll into smooth balls if desired (if the batter becomes too soft, chill the portioned scoops of batter before rolling). Top with chocolate chips and salt flakes then bake for 8-10 minutes until crackled on top but still soft. Let cool completely on the trays.