Brown the Butter: Melt the unsalted butter in a medium saucepan over medium heat, stirring frequently for about 5-7 minutes until golden brown.

Mix Wet Ingredients: In a large mixing bowl, combine the cooled brown butter with dark brown sugar and white sugar. Beat until smooth, then add the egg, egg yolk, and vanilla extract, mixing until creamy.

Combine Dry Ingredients: In a separate bowl, whisk together flour, espresso powder, baking soda, and salt until evenly mixed.

Mix Together: Gently fold the dry ingredients into the wet ingredients until just combined, then fold in dark chocolate chunks.

Chill the Dough: Wrap the dough in plastic wrap and refrigerate for at least 30 minutes, or up to overnight.

Preheat Oven: Preheat your oven to 350°F (175°C).

Bake Cookies: Scoop chilled dough onto lined baking sheets, spacing about 2 inches apart. Bake for approximately 11 minutes, until edges are golden brown.

Cool: Allow cookies to cool on baking sheets for 5-7 minutes before transferring to a wire rack to cool completely.