In a saucepan over medium heat, melt the butter. Cook, stirring frequently, until the butter foams and the milk solids turn golden brown and smell nutty (*see notes). Pour into a large mixing bowl and immediately stir in the milk powder(**see notes) and espresso powder if using. Let the brown butter sit for at least 10 minutes. It will be cool enough to touch.

Add the brown sugar and granulated sugar to the cooled brown butter. Whisk for 1 minute until the mixture becomes smooth and glossy.

Whisk in the whole egg, egg yolk, and vanilla extract or vanilla bean paste until fully combined.

Add the flour, cornstarch, baking soda, baking powder, and salt. Fold until a soft dough forms with some flour streaks remain. Add chopped chocolate.

Scoop large portions of dough about a 1/4 cup (85 g / 3 oz) of dough per cookie and place on a cookie sheet and top each dough ball with a piece of chocolate for bakery-style presentation. Cover the dough with plastic wrap and refrigerate for at least an hour up to 48 hours for the best flavor. This step allows the flour mixture to hydrate and develops deeper flavor in the cookie dough. If you're short on time, the cookies can be baked right away, but chilling will give the best flavor.

Bake: Once ready to bake preheat oven to 350°F (175°C). Place the cookie dough balls onto a parchment-lined baking sheet, leaving several inches between each cookie. Bake for 10–12 minutes, until the edges are golden brown and the centers still look slightly underbaked and puffy. Sprinkle each cookie lightly with flaky sea salt. These cookies will continue to bake and set as they cool. Leave the baked cookies on the cookie sheet for 5 mins and then transfer to a cooling rack for at least 30 mins.