Brown the butter by melting unsalted butter in a sauté pan over medium heat until it turns golden brown, approximately 5-7 minutes.

In a bowl with the cooled browned butter, combine granulated sugar and dark brown sugar, mixing well until fully incorporated.

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

Whisk in large eggs and vanilla extract into the cooled sugar mixture until uniform.

Gradually add dry ingredients to wet ingredients using a rubber spatula, mixing just until combined.

Wrap the dough in plastic wrap and refrigerate for 24 to 72 hours.

Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.

Scoop balls of dough using a 3-tablespoon scoop, placing them at least 2 inches apart on the lined sheets.

Bake the cookies for 12-14 minutes until golden around the edges but soft in the center.

Immediately sprinkle with flaky sea salt and cool on baking sheets for 2 minutes before transferring.