In a medium bowl, whisk the heat-treated flour, cornstarch, and salt together. Set aside. Using a handheld or stand mixer fitted with the paddle attachment, beat the butter on medium speed in a large bowl until smooth and creamy, about 2 minutes. Add the brown sugar and mix on medium until light and fluffy, about 3 full minutes. Using a silicone spatula, scrape down the sides and up the bottom of the bowl as needed. Add the egg whites and vanilla extract and mix until fully incorporated, about 2 minutes.

Add half of the flour mixture and mix on low just until the powdery texture of the flour disappears, about 15 seconds. Immediately add the rest of the flour mixture and mix until combined. Remove the bowl from the mixer and use a rubber spatula to fold in the chocolate chips.

Freeze or refrigerate the cookie dough as you prepare the buttercream portion of the recipe.

Using a handheld or stand mixer fitted with the paddle attachment, beat the butter in a large bowl on medium speed for 3 minutes, or until light, fluffy, and completely smooth. Add the vanilla extract, milk, and salt and beat until combined. Using a silicone spatula, scrape down the sides and up the bottom of the bowl as needed. Add the confectioners’ sugar, then beat on low speed for 15 seconds. Switch to medium-high speed and beat the frosting until super light and fluffy, about 4-5 minutes. The frosting will become lighter in color as air is whipped into it.

Break the cookie dough into quarter-size chunks. (If desired, save enough dough to make little cookie dough ball garnishes for the top of your cake/cupcakes—or save some for snacking, there’s plenty of cookie dough!) With the mixer running on medium speed, add the cookie dough chunks. Mix just until incorporated—some chunks can remain.

You can spread the frosting onto a cake or cupcakes with a knife or icing spatula or apply with a large round piping tip. (The cookie dough chunks will get stuck in a smaller tip.)

Use immediately or cover tightly and store for up to 1 week in the refrigerator or up to 3 months in the freezer. After freezing, thaw in the refrigerator then beat the frosting on medium speed for a few seconds so it’s creamy again. Adding a splash of heavy cream or milk will help thin the thawed or refrigerated frosting out again, if needed.