Cream the butter, brown sugar, and granulated sugar in a stand mixer until smooth and light. Beat in the egg until combined.
Add the flour, cornstarch, baking soda, and salt, mixing just until incorporated. Fold in the chopped chocolate, chocolate chips, crushed Biscoff cookies, and Biscoff spread.
Shape the dough into balls and chill overnight or freeze for 1 hour. Arrange four dough balls on the prepared baking sheet, and press a few extra chocolate pieces and Biscoff chunks on top.
Bake on a silicone mat in a 350°F/175°C oven for 9–12 minutes, until the edges are set and the centers are soft.
Sprinkle with flaky salt and cool completely on a wire rack.
For a richer, gooier center, press a square of dark chocolate inside each dough ball before baking.