Preheat your oven to 350°F and line two baking sheets with parchment paper.
In a large mixing bowl, whisk together the melted butter, dark brown sugar, and granulated sugar until smooth and glossy.
Add the egg, egg yolk, and vanilla extract. Whisk until fully combined and creamy.
In a separate bowl, whisk together the flour, cocoa powder, espresso powder, baking soda, and kosher salt.
Gradually fold the dry ingredients into the wet ingredients using a spatula until just combined. Be careful not to overmix.
Fold in the chopped chocolate chunks, saving a handful for topping the cookies before baking.
Scoop the dough into large cookie dough balls and place them on the prepared baking sheets, leaving space between each cookie.
Place the cookie dough balls in the freezer for 30 minutes to chill.
Bake at 350°F for 12 minutes, or until the edges are set and the centers still look slightly soft.
Remove from the oven and immediately press extra chocolate chunks into the tops of the cookies if desired. Sprinkle with flaky sea salt.
Let the cookies cool on the baking sheet for about 5 minutes before transferring to a wire rack to cool completely.
