In a large mixing bowl, cream the butter and sugars with an electric mixer on medium speed until light and fluffy. Add the eggs one at a time, followed by the vanilla extract. Scrape the bottom and sides of the bowl, then blend again until combined.

In a separate bowl, whisk the flour, cocoa powder, baking soda, salt, and espresso powder, if using.

Gradually add the dry ingredients to the wet ingredients and mix until almost combined. Stir in 2 tablespoons of milk and 1 1/2 cups of chocolate chips. If the dough is very thick, add more milk, as needed. It should be thick but scoopable.

Cover and chill the dough for at least 3 hours (or overnight) to allow the flavors to meld.

Let the dough soften at room temperature for 10-20 minutes, then preheat the oven to 350°F. Line two baking sheets with parchment paper.

Scoop the dough into 2-3 tablespoon-sized balls and place on the prepared baking sheet, leaving about 3 inches of space between each cookie. Press the remaining chocolate chips onto the tops and sprinkle with sea salt, if using.

Bake for 12-15 minutes or until the edges are set and the centers are slightly soft.

Remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. Serve and enjoy!