Add the room-temperature egg, egg yolk, and instant espresso powder to the butter mixture. Beat on medium speed until smooth and fully combined, making sure the instant espresso powder dissolves completely.
Add the all-purpose flour and baking soda to the bowl. Switch to a rubber spatula and gently fold the ingredients together just until almost combined.
Add the chocolate chunks and fold gently until evenly distributed. Do not overmix!
Using a large cookie scoop (about 3 tablespoons per ball), scoop the soft coffee cookie dough into balls and place them onto your prepared baking sheet.
Cover the tray loosely with plastic wrap and place in the refrigerator to chill for at least 1 hour (or overnight).
Preheat your oven to 180°C (350°F). Space the chilled coffee cookie dough balls at least 2 inches apart on your baking sheets.
Bake for 10 to 12 minutes, until the edges are golden brown and set, but the centers are still soft. Immediately press a few extra chocolate chunks into the tops of the warm cookies for a bakery-style finish.
Let the coffee cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely. I like to top them with flaky salt!
Notes
Espresso Powder: Use instant espresso powder or instant coffee granules only. Do not use actual ground coffee or espresso beans, as they will not dissolve and will give the cookies a gritty texture.
Measuring Flour: Always use the spoon and level method to measure flour. Fluff the flour with a spoon, scoop it into your measuring cup, and level it off with the back of a knife. Scooping directly with a measuring cup packs the flour down and leads to dry, cakey cookies.
Scoop Before Chilling: Always scoop the soft dough into balls immediately after mixing. Chilling the entire bowl of dough first makes it rock hard and extremely difficult to scoop later.
Chill Time: Chill the pre scooped dough balls for at least 1 hour before baking. Cold butter slows down the spreading process in the oven, keeping your coffee cookies thick with soft, chewy centers.
Baking Time: Bake until the edges are set and lightly golden, but the centers still look slightly soft and underbaked. The cookies will continue to cook and set on the hot baking sheet after coming out of the oven.
Bakery Style Finish: Press a few extra chocolate chunks into the tops of the cookies immediately after taking them out of the oven, and sprinkle with a pinch of flaky sea salt if desired.
Storage: Keep baked cookies in an airtight container at room temperature for up to 4 days. Store with a slice of white bread in the container to retain maximum moisture and softness.