Preheat oven to 325 degrees. Line 2 baking sheets with parchment paper; set aside.

2.In a medium bowl, whisk together flour, cocoa powder, salt, and baking soda; set aside. In a small bowl, stir together espresso powder, vanilla, and 2 teaspoons of water; set aside.

3.In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugars together on medium speed until light and fluffy, about 5 minutes. Add egg and yolk; mix until incorporated. Add espresso mixture; mix well, scraping down the sides of a bowl with a spatula. At low speed, add flour mixture in small batches, mixing until just combined. Remove bowl from mixer, and stir in chocolate chunks.

4.Using a 2-ounce ice-cream scoop, scoop heaping balls of dough and place 3 inches apart on prepared baking sheets. Bake until cookies are set, about 20 minutes. Remove from oven; let cool completely before removing cookies.