Cut cold butter into cubes. In a stand mixer with a whisk attachment, or a large bowl with a hand mixer, cream together butter and brown sugar for 3–4 minutes until the consistency is similar to peanut butter.
Beat in eggs and vanilla. Then add cornstarch, baking soda, salt, and cocoa powder, mixing until just combined. Gradually mix in cake and all-purpose flour until no streaks remain. Avoid overmixing. Fold in chocolate chips. Cover and refrigerate the dough for at least 1 hour or overnight.
If dough was chilled longer than 1 hour, let it soften slightly at room temperature. Roll dough into 2 tablespoon-sized balls. Chill formed cookie balls in fridge for 10 minutes.
Preheat oven to 325°F. Space cookies 2 inches apart on a sheet pan lined with parchment paper or a silicone baking mat. Bake for 8–12 minutes. Lean toward underbaking for fudgy cookies (see note 4). Upon removal, immediately press wayward edges inward with a spatula. Optionally, add extra chocolate chips on top and a light sprinkle of sea salt. Let cookies rest on the sheet for 5 minutes before transferring to a cooling rack.
Notes
Note 1: Use Dutch-process cocoa powder like Hershey’s® Special Dark for optimal flavor and moisture. Regular cocoa powder may result in a less flavorful, drier cookie. Measure by spooning into the cup and leveling off; excess can dry out cookies.Note 2: Cake flour is essential for a fluffy, tender crumb. Don’t substitute and don’t try to make a DIY cake flour; it doesn’t work in this recipe.Note 3: Milk chocolate is my preference, but feel free to use semi-sweet or dark chocolate!Note 4: If cookies aren’t spreading by 7–8 minutes, take them out of the oven and tap the baking sheet 2–3 times on the counter to encourage spreading. Then, return them to the oven to finish baking.Storage: Store in an airtight container at room temperature for up to a week. Rewarm leftovers in microwave. You can also freeze dough balls instead of storing baked cookies. Bake from frozen, adding extra time as needed.