In a medium bowl, whisk together the flour, espresso powder, cornstarch, baking soda, and salt until combined. Set aside.

Using a separate mixing bowl, cream together the softened butter and sugars on medium-high speed until the mixture is light and fluffy, about 2 minutes.

Add egg and vanilla, and beat on medium speed until combined, about 30 seconds. Add the dry ingredient mixture, and beat on medium-low speed until just combined. Fold in the chocolate chips by hand until evenly combined.

Transfer the dough to an airtight container and refrigerate for 2 hours, or until chilled through.

Preheat oven to 350?F and line a baking sheet with parchment paper.

Once the cookie dough is chilled, scoop out and roll dough balls that are 1-inch in diameter and place them on the prepared baking sheet.

Bake for about 10-12 minutes, or until edges are slightly crispy and the tops are lightly golden. The centers may look slightly undercooked.

Remove the cookies and let them cool on the baking sheet for about 5 minutes. Sprinkle with a pinch of flaky sea salt if desired.

Serve warm.