Preheat oven to 325°F (165°C) and line an 8×8-inch pan with parchment paper.
Melt butter in a saucepan until it turns golden brown and smells nutty, then pour into a bowl.
Mix powdered sugar into warm butter until it forms a thick paste. Let it cool slightly.
Whisk in eggs, egg yolk, and vanilla until smooth and glossy.
Add cocoa powder, then fold in flour, malted milk powder, cornstarch, and salt.
Stir in chocolate chips. Batter will be thick.
Spread batter evenly into prepared pan.
Bake for 30–33 minutes until set with moist crumbs on a toothpick.
Cool completely before slicing and sprinkle with flaky sea salt if desired.