Preparation Steps

• Whisk together the flour, cornstarch, milk powder, baking soda, salt, and espresso powder in a large bowl; set aside.
• In a stand mixer fitted with the paddle (or using a hand mixer), beat the butter, brown sugar, and granulated sugar on medium-high until very fluffy and a few shades lighter, 3 to 4 minutes. Scrape the bowl.
• Mix in the egg, egg yolk, and vanilla on medium speed just until smooth and glossy, about 30 to 45 seconds. Do not overmix after the eggs are added.
• Add the dry ingredients in two additions on low speed until a thick dough forms with a few dry streaks remaining. Finish by folding with a spatula until no flour remains. Fold in the chocolate chips.
• Let the dough rest 10 minutes at room temperature, then refrigerate for 15 to 20 minutes to slightly firm up. During this chill, position a rack in the center of the oven, preheat to 365°F (185°C), and line two baking sheets with parchment paper.
• Portion the dough into 12 mounds of about 4 ounces (113g) each. Stack each portion into a tall, slightly rough cylinder to encourage height and a thick center. Arrange 6 per sheet, leaving space to spread.
• Bake one tester cookie first to gauge your oven: 14 to 16 minutes, or until the edges are set and deep golden and the tops look puffed and matte (the centers should still feel soft). Adjust time as needed, then bake the remaining cookies.
• If you want perfectly round cookies, immediately scoot a round cutter around each hot cookie to tidy the edges. Cool on the sheet for 10 minutes, then transfer to a rack to finish cooling.
• Bake the second tray on a fully cooled sheet to prevent excess spread.