Gather your mise en place; get together all of the equipment needed for the recipe and gather your ingredients. Preheat the oven to 375°F and line baking sheets with parchment paper.

Place the butter in a large skillet over medium heat. Stir frequently as the butter melts and begins to foam. Continue cooking until the butter turns golden brown and small brown specks form on the bottom of the pan, about 5 – 7 minutes. Immediately transfer the butter to a heatproof bowl and allow it to cool until warm but no longer hot.

In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt.

Pour the cooled brown butter into a large mixing bowl. Add the granulated sugar and brown sugar and whisk until well combined. Add the egg, egg yolk, vanilla, and milk, mixing until smooth.

Add the dry ingredients to the wet ingredients and stir just until no dry flour remains. Fold in the chocolate chunks.

Using a cookie scoop or approximately 2 tablespoons of dough, portion the cookies onto the prepared baking sheets, leaving about 2 inches between each cookie. If desired, sprinkle each cookie with a small pinch of flaky sea salt.

Bake for 10 – 12 minutes, or until the edges are lightly golden and the centres still appear slightly soft. Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a cooling rack.

Serve warm or allow to cool completely before storing in an airtight container.