Brown the butter in a saucepan over medium heat until golden and nutty, 3–4 minutes. Remove from heat.
Stir the chopped dark chocolate into the warm butter until glossy, then stir in both sugars. Cool for 2 minutes.
In a separate bowl, whisk the eggs, extra yolk, vanilla, espresso powder, salt, and baking powder until combined.
Pour the chocolate-butter mixture into the eggs and whisk hard for 1 minute, until smooth and slightly thickened.
Sift in the cocoa and flour; fold gently until just combined. Do not overmix.
Fold in the chocolate chips.
Pour into a lined tin that fits inside the Ninja Crispi; scatter flaky salt on top.
Bake on the ReCrisp setting (170°C) for 22–25 minutes, until a skewer shows a few moist crumbs.
Cool fully before slicing. For clean squares, chill 20 minutes and wipe the knife between cuts.