In a medium sauté pan, melt the butter over medium heat, stirring occasionally. Keep cooking until it turns a nutty brown color with small brown bits at the bottom, which takes about 5-7 minutes.

In a mixing bowl, combine the browned butter with the granulated sugar and dark brown sugar. Stir well until the sugars are fully incorporated. Allow the mixture to cool to room temperature, about 10-15 minutes.

In a separate bowl, whisk together the all-purpose flour, bread flour, baking soda, baking powder, fine sea salt, and instant espresso powder, if using.

Once the butter-sugar mixture has cooled, whisk in the eggs, egg yolk, and vanilla extract until well combined. Gradually mix in the dry ingredient mixture using a rubber spatula.

Wrap the cookie dough tightly in plastic wrap and refrigerate for a minimum of 24 hours, or up to 72 hours.

Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.

Using a cookie scoop or your hands, portion out approximately three tablespoon-sized balls of dough onto the prepared baking sheets, spacing them at least 2 inches apart.

Place the cookie sheets in the preheated oven and bake for 12-14 minutes, or until the edges are golden brown and the centers are still slightly soft.

Immediately after removing the cookies from the oven, use a round cookie cutter slightly larger than the cookies to gently reshape them.

Sprinkle the warm cookies with flaky sea salt for a delightful finishing touch, and let them cool on the baking sheets for about 5 minutes before transferring them to wire racks.