OREO PREP: Take 8 whole Oreos and coarsely chop them into small bite-sized pieces. Measure to get 1 cup of cookie pieces and set aside. For the frosting: take 6 whole Oreos and blend in a high-powered blender or food processor until you have fine crumbs. Set aside.
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.
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 (Read Note 4 first) and regular flour and again beat until combined. Don't over-beat/mix the mixture. Add in the miniature, regular-sized chocolate chips, and coarsely chopped Oreos. Fold the chips and cookies the dough with a spatula. Cover the bowl tightly and refrigerate for 45 minutes up to 2 hours.
FORM COOKIE BALLS: After chilling, 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.
BAKE: Meanwhile, preheat the oven to 325 degrees F. Line an extra-large sheet pan with a Silpat liner (parchment paper works, but the cookies bake better on a Silpat liner). Add 3 cookie dough balls to the pan, leaving plenty of space in between each cookie. Bake for 13-18 minutes, erring on the side of slightly under-baking (we think they're perfect at 15 minutes). They're slightly under-baked but will firm up to a nice and fudgy/chewy cookie as they set. Remove from the oven and immediately press the edges of the cookie inwards with the back of a metal spatula to get the perfect round bakery-style cookie. Let cookies stand on the 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.
FROSTING (See Note 3): In a large bowl (or stand mixer fitted with whisk attachment), beat the butter until smooth, creamy, and light, about 3-4 minutes. Add in 1 cup of powdered sugar and beat until combined. Beat in the remaining 1 cup powdered sugar, vanilla, and salt, and then just enough heavy cream to get the frosting to a pipeable consistency (we like 3 tablespoons, but you may want a touch more). Stir in the crushed Oreo crumbs until integrated. Transfer to a piping bag fitted with a 1M Open Star piping tip and generously pipe frosting on each cookie. Alternatively, spread on frosting with a knife. (We use all the frosting between the 6 cookies -- these are supposed to be indulgent bakery-style cookies after all. :) If you don't want to use all the frosting on the cookies, frost as much as you'd like on cookies and save the rest to use as a fruit dip or addition to other desserts.
OPTIONAL GARNISH: For the perfect finishing touch, add a couple of mini Oreo cookies, halved regular-sized Oreos, or some coarse Oreo crumbs (just break with your hands) to the top of the cookies.
STORAGE: Because of the frosting, these cookies don’t store or freeze well. If you’re going to have extra cookies that won’t be enjoyed the same day, I recommend storing the frosting and cookies separately and frosting right before serving. The frosting needs to be refrigerated in an airtight container, while the cookies do best when stored in an airtight container at room temperature. Allow the frosting to soften/come to room temperature for about 30 minutes before frosting the cookies; it’s very stiff right out of the fridge.