Melt the chopped chocolate and butter together in a large microwave-safe bowl in 30 second bursts, stirring each time, until smooth. Whisk in the brown sugar and granulated sugar, then whisk in the milk and vanilla until glossy.
In a separate bowl, whisk the flour, cocoa powder, cornstarch, baking powder and salt. Fold into the chocolate mixture until a soft, thick dough forms, let it cool for 10 minutes, then fold in the chocolate chips.
Cover the bowl and refrigerate for 45 minutes, until the dough is firm enough to scoop and hold its shape.
Near the end of the chilling time, preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
Scoop the dough into 1 1/2 tablespoon balls and place them 2 inches apart on the prepared baking sheets.
Bake one sheet at a time for 10 to 11 minutes, until the edges are set and the tops look slightly cracked but the centers are still soft. Sprinkle with flaky sea salt right away.
Let the cookies cool on the baking sheet for 10 minutes, where they finish setting into that fudgy brownie texture, then move them to a wire rack. Makes about 18 cookies.