Line a 9×9-inch baking pan with parchment paper or aluminium foil, leaving enough overhang on the sides to easily remove the mixture once it has set. You need a baking pan to make these cookies because rather than rolling the dough into balls, it is spread into a pan, then cut into squares.
Place 210g bittersweet chocolate and butter in a large bowl and set it over a saucepan of simmering water, stirring occasionally, until melted and smooth. Remove from heat. Alternatively, melt in the microwave. Add the sugar and mix, then add the eggs, and mix until combined.
In a small bowl combine cocoa powder, cornstarch, baking powder, and salt. Add it to the chocolate mixture and whisk just until combined. Stir in 115 g chopped chocolate. Spread mixture evenly into prepared pan. Cover and refrigerate for at least 2 hours or until firm.
Preheat oven to 300F/150C. Line 2 large baking sheets with parchment paper.
Lift the mixture out of the pan using the parchment paper and cut into 36 squares. Place the squares on the baking sheets 2-inch/5cm apart (there's on need to shape them into balls).
Bake for 10-12 minutes. The cookies should stay soft and brownie-like in the center. Overbaking will make them dry. Allow cookies to cool on a wire rack.