Brown the butter. In a medium saucepan, add the butter pieces. Melt the butter over medium heat, stirring occasionally. As it melts, the butter will begin to bubble and foam. Continue stirring until the bubbling stops, the butter turns an amber color, and you see darker bits at the bottom, releasing a nutty aroma. This process will take a few minutes. Remove from heat and let it cool to room temperature. Tip: Use a lighter-colored saucepan so you can easily monitor the butter’s color.

Line two baking sheets with parchment paper or a silicone baking mat. Preheat the oven to 350°F.

In a medium-sized bowl, combine the sifted flour, baking powder, baking soda, and salt. Set aside.

In a separate, larger bowl, combine the cooled brown butter, granulated white sugar, and brown sugar. Beat with an electric mixer until combined.

Add the egg, egg yolk, milk, and vanilla, and beat until the mixture is smooth.

Slowly add the flour mixture to the wet ingredients, folding gently with a spatula just until combined. Fold in ¾ cup of chocolate chunks. Save the rest to decorate the tops.

Form dough balls using 2 tablespoons of dough. Place them on the prepared baking sheet, spacing them about 2 inches apart. Add a few extra chocolate chunks on top of each dough ball. Gently press the balls to slightly flatten them.

Bake for 10-12 minutes, or until the edges are lightly golden. The center may seem slightly underbaked, and that's okay. It will continue to bake on the hot baking sheet once you remove them from the oven. Sprinkle with flaky sea salt. Let them rest on the baking sheet for 5 minutes, then transfer to a cooling rack to cool completely.