Heat Treat Flour: See note 1; You can heat treat flour in the microwave or oven. Test the flour to ensure it’s reached a safe temperature of 165°F. Let flour cool completely to room temperature before using. Don’t use any burnt or clumpy flour (if it’s off color or smells burnt, it is burnt). The flour should be light, white, and fluffy. Use a spoon to measure the cooled flour into a measuring cup and level the top of the cup with the back of a table knife.
Line a 9x9-inch pan with parchment paper, leaving an overhang of paper so you can easily remove the bars to cut and serve.
Place the melted butter in a large bowl and let it cool to room temperature. Hot butter will melt the sugars and cause greasy dough. Once cooled, add the brown sugar and granulated sugar. Stir to combine. Add milk, vanilla, salt, and heat-treated flour. Stir with a wooden spoon until dough is smooth. Stir in mini chocolate chips until combined (you may need to knead the chocolate chips in by hand). If the dough is too dry, add a bit more heavy cream; if it is at all wet, add 1–2 tablespoons more heat-treated flour.
Press the prepared cookie dough evenly along the bottom of the pan. Use the bottom of a 1-cup measuring cup to press the dough firmly into an even layer. Scoop the cookie dough ice cream over the cookie dough layer. Use a strong spatula to smooth the ice cream as much as possible into an even, flat layer. Cover and freeze bar for 2 hours up to overnight.
In a bowl attached to a stand mixer with a whisk attachment, add the heavy cream, powdered sugar, and vanilla extract. Beat to combine, then increase the speed. Whip until firm peaks form, about 2–4 minutes. With a spatula, spread the whipped cream over the ice cream until smooth. Alternatively, spread 1 (8-ounce) container fully thawed whipped topping on top of the bars. Freeze bars to harden, about 30 minutes to an hour.
To serve, remove from the pan using the parchment paper overhang and cut into bars. If desired, top each bar with a handful of crumbled cookies. You can also add hot fudge topping and/or mini chocolate chips if desired. (I usually just serve with cookie crumbs.)
Notes
Note 1: Here’s how to heat treat flour:
Heat treat 1/2 cup more flour than you’ll need (in case edges burn).
Microwave: Add flour to a microwave-safe bowl and microwave in 30-second bursts, stirring between each, to avoid burning or clumping. After each burst, check the temperature. It’s done when it has reached 165°F.
Oven: Preheat the oven to 300°F and line a large sheet pan with a nonstick liner or parchment paper. Spread the flour on the pan. Bake, stirring and checking the temperature every 2 minutes until it reaches 165°F, about 3–6 minutes.
Note 2: Either use 1 (8-ounce) container of whipped topping (completely thawed) like Cool Whip® or make your own whipped topping as explained in the recipe. I prefer homemade, but either works great!Note 3: Optional toppings: 3–4 chocolate chip cookies to crumble on top, hot fudge topping or chocolate sauce, and mini chocolate chips.Storage: Return any leftovers promptly to the freezer. Whipped topping doesn’t thaw and refreeze well (it becomes icy), so try to finish the bars a few days after making.