CREAM BUTTER AND SUGAR: Cut butter straight from the fridge into small cubes. In a large bowl (or stand mixer fitted with whisk attachment) add in the cubed butter and brown sugar. Beat until completely combined and ultra creamy, about 3-4 minutes. Mixture should be the consistency of peanut butter.
ADD REMAINING INGREDIENTS: Add in the egg and vanilla. Beat until combined. Add in the cornstarch, baking soda, salt, and cocoa powder. Beat until just combined, scraping down the sides with a spatula as needed. Add in the cake flour and regular flour (Note 3) and again, beat until combined. Don't overbeat/mix the mixture. Add in the miniature and regular-sized chocolate chip. Fold the chips into the dough with a spatula. Cover the bowl tightly and refrigerate for 45 minutes up to 2 hours.
FORM COOKIE BALLS: After 45 minutes, remove the bowl from the fridge and form cookie dough balls. Each ball should be just over 4 ounces (117 grams) each. You should get 6 equal-sized balls from the dough. Roll into large balls and place on a parchment-paper-lined sheet pan or plate and return to the fridge or freezer to chill for 15 more minutes. (Note 5) Want to make smaller cookies? See Note 7
BAKE: Meanwhile, preheat the oven to 325 degrees F. Line an extra-large sheet pan with a Silplat liner (parchment paper works, but cookies bake even better on a silicone liner). Add 3 cookie dough balls to the sheet pan, leaving plenty of space in between each cookie. Bake for 13-18 minutes, erring on the side of slightly underbaking (we think they're perfect at 15-16 minutes). They're slightly underbaked but will firm up to a nice and fudgy/chewy cookie as they set (Note 6.) Remove from the oven and right out of the oven, press edges of the cookie inwards with the back of a metal spatula to get the perfect round bakery-style cookie. If desired, press a few extra chocolate chips into the tops of the cookies right out of the oven. Let cookies stand on the cookie sheet pan for 10-15 minutes before removing them to a cooling rack. Allow cookies to cool and finish firming up here. Repeat process with the last 3 cookies.
STORAGE: We like these cookies best the day they're baked. They do last up to a week, but they begin to lose texture and flavor after 3-4 days. To store: Place in an airtight container and keep at room temperature. Wait until cookies are completely cooled before adding to the container. Rewarm in increments of 10-15 seconds in the microwave.
FREEZING: Instead of freezing the baked cookies, freeze the dough! Drop the cookie dough balls on a large sheet pan and freeze until solid. Once solid, transfer the frozen cookie dough balls to an airtight container or bag and freeze for up to three months. To bake: You can bake these cookies straight from the freezer. There is no need to thaw, but you may need to add a few extra minutes to the baking time. Bake until the edges are lightly browned, and the center is still soft.