In a medium bowl, add the flour, cocoa powder, baking soda, baking powder, and cornstarch. Whisk together then set aside.: The aroma of the cocoa powder becomes more pronounced as you whisk, and you will see a uniform dark blend with tiny pale specks from the cornstarch . Whisking aerates the dry mix slightly, which helps distribute leavening evenly so cookies rise consistently. A useful sensory tip is to watch for an even color with no streaks of flour or clumps of cocoa. If you skip thorough whisking you might get uneven pockets of rising that create misshapen cookies, so take the extra minute to sift or whisk well.
In a large mixing bowl, beat together the butter, granulated sugar, and brown sugar until light and fluffy (about 2 minutes on medium speed). Add vanilla extract and eggs and beat together until combined. Use a rubber spatula to scrape the sides of the mixing bowl.: When creaming, you will notice the texture shift from grainy to smooth as the butter and sugars incorporate. The mixture should look paler and slightly voluminous, with a gentle sheen. Adding the eggs and vanilla extract brings the batter together into a glossy emulsion. If you hear the mixer clanging against the bowl, lower the speed to keep aeration controlled. A common mistake here is undercreaming, which can lead to dense cookies, or overbeating, which can incorporate too much air and cause excess spread; aim for that pale, light texture.
Turn the mixer on low and gradually add half of the dry ingredients to the butter mixture. Beat until combined. Add half of the heavy cream and mix to combine. Add in the remaining dry mixture and the remaining cream.: Adding dry ingredients in portions prevents flour clouds and helps maintain a tender crumb. You will feel the batter thicken as you fold in the dry mix, and the sound of the mixer will change to a softer whir. The small amount of heavy cream loosens the dough slightly, making it pliable without making it wet. If you add all the dry ingredients at once you risk overmixing while trying to incorporate them, which can develop gluten and create tougher cookies. Watch for a cohesive dough that pulls away from the bowl, not a sticky batter.
Beat in the chocolate chips just until combined.: When you fold in the semi-sweet chocolate chips the dough will litter with shiny chips, promising molten bites. Avoid overmixing at this stage to keep chunks intact and pockets of chocolate dispersed. The chips provide visual contrast and bursts of texture; be sure they are evenly distributed so each cookie gets a good share. Overworking this step can break chips and warm the dough, causing excessive spread later in the oven.
Cover the dough with plastic wrap and chill for 1 hour in the refrigerator.: Chilling firms the fats and allows flavors to meld, producing a cookie that spreads less and has a chewier center. You will feel the dough become firmer to the touch after chilling, which makes scooping cleaner and rolling smoother. This hour also helps hydrate the flour for better structure. Skipping chilling is a frequent shortcut that leads to flat, thin cookies, so plan ahead and give the dough this rest.
Preheat the oven to 350 degrees and line baking sheets with parchment paper.: As the oven warms you will notice a gentle toasty scent that will complement the baked cookies. Lining trays with parchment ensures even browning and makes cleanup simple. Use an oven thermometer if you suspect your oven runs hot or cold. Placing cookies directly on an unlined sheet can cause uneven browning and sticking, so always prepare the pan properly.
Use a cookie scoop to make balls from the chilled dough. Roll each ball between the palms of your hands for a smooth cookie. Space 2" apart on prepared cookie sheets.: Rolling creates a taut surface that encourages an even dome while baking, and spacing allows air to circulate so cookies bake uniformly until edges set and centers remain soft. You will feel the chilled dough warm slightly as you roll, which is normal; work quickly to keep the dough cold. Crowding the pan is a common mistake, and it leads to merged cookies and unpredictable bake times, so respect the spacing guideline.
Bake for 9 minutes and remove from the oven. Sprinkle several additional chocolate chips over the tops of the warm, baked cookies if desired.: During baking you can watch edges set and centers remaining glossy; nine minutes typically yields a set edge with a soft center. Once out of the oven the surface will look slightly underbaked, which is perfect because carryover heat finishes the center without drying it out. Pressing extra chocolate chips onto warm cookies adds glossy, melty appeal. Leaving cookies in the oven too long is the usual mistake here, resulting in dry, crumbly textures, so err on the shorter side.
Transfer cookies to a cooling rack and allow them to cool.: Moving cookies off the hot sheet is crucial because residual heat will continue to cook their bottoms if left on the pan. On the rack they cool evenly and the bottoms firm just enough while centers retain chew. You will hear a soft settling sound as steam releases, and the scent of chocolate intensifies as they cool. Rushing to stack them while still hot can lead to smashed shapes or melted chips sticking together, so patience pays off.