Place the butter in a small saucepan, and place over medium heat. Cook until the butter has melted, and then continue to cook, stirring constantly, for 4 - 5 minutes. The butter will foam on top and turn golden brown and nutty underneath the foam - keep an eye on it, it will burn quickly if left unattended. 

Remove the butter from the heat and let cool slightly before putting in a heatproof glass container. Place in the refrigerator for 2 hours, or until butter solidifies, but it still soft.

Preheat oven to 350°F / 325°F convection / 180°C. Line a baking sheet with parchment paper or a silicone baking mat. Set aside.

With an electric mixer, beat browned butter with water, sugar and brown sugar until fluffy, 2 - 3 minutes.

Add egg and vanilla. Beat until combined.

In a medium mixing bowl, whisk flour, cocoa powder, baking soda, salt and espresso powder (if using) together until thoroughly combined.

Add dry ingredients to sugar mixture and mix until dough looks almost uniform, with a few pockets of flour.

Add chips and mix just until combined - 30 seconds or so, don't overmix.

Use a 2 ounce scoop or a 1/4 cup measure to portion 10 large cookies onto prepared baking sheet. Lightly flatten the tops and place a few more chips on top, if desired.

Bake for 18 minutes, rotating halfway though baking time. Let cookies cool on baking sheet for 5 minutes before moving to a cooling rack to cool completely.