In a large bowl, whisk the slightly cooled melted butter, brown sugar and granulated sugar for about 1 minute, until smooth and glossy. Whisk in the milk and vanilla until fully combined.
In a separate bowl, whisk together the flour, cocoa powder, cornstarch, baking soda, espresso powder and salt. Break up any cocoa lumps so they don't end up in the cookies.
Add the dry ingredients to the wet mixture and stir with a spatula until no dry streaks remain. The dough will be thick and fudgy.Fold in 1 cup of the chocolate chips.
Cover the bowl and chill the dough for 30 minutes. This firms up the melted butter so the cookies bake up thick instead of spreading flat.Near the end of chilling, preheat the oven to 350°F and line two baking sheets with parchment paper.
Scoop the dough into 2 tbsp balls and place them 2 inches apart on the prepared sheets. Press the remaining 1/4 cup chocolate chips onto the tops.
Bake for 10-11 minutes, until the edges are set and the centers still look soft and slightly underbaked. Sprinkle with flaky sea salt right away, if using.
Let the cookies cool on the baking sheet for 10 minutes so the centers finish setting, then transfer to a wire rack. Makes about 16 cookies.