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 silicone spatula to fold in the chocolate chips.

Freeze or refrigerate the cookie dough for 15 minutes or until the dough is easy to handle. Roll the dough into 12 one-Tablespoon-size balls, place on a plate lined with parchment paper, then refrigerate until step 9. Cover the remaining cookie dough and set aside at room temperature until step 11.

Preheat the oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners.

Whisk the flour, baking powder, baking soda, and salt together in a medium bowl. In a small bowl, whisk the milk and sour cream together until smooth. Set aside.

Using a handheld or stand mixer fitted with the paddle attachment, beat the butter and sugar together on medium speed in a large bowl until light and fluffy, about 3 minutes. Using a silicone spatula, scrape down the sides and up the bottom of the bowl as needed. Add the egg whites and vanilla and beat for 2 minutes, or until the mixture is totally incorporated. With the mixer on low, alternate additions of the flour mixture and the sour cream mixture, starting and ending with flour. Beat for an additional 30 seconds or until the batter appears smooth and creamy.

Fill cupcake liners about 3/4 full. Bake for 16-19 minutes or until the cupcake edges are golden brown and a toothpick inserted in the center comes out clean. Remove from the oven and let cool in the pan for 5 minutes.

Remove cupcakes from the pan and carefully press 1 chilled cookie dough ball into the center of each cupcake. The dough ball might not sit completely flat, so expect a little lump on top. Don’t worry, you’ll cover it with frosting. Cool cupcakes completely.

While the cupcakes cool, make the cookie dough buttercream: 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 remaining cookie dough into quarter-size chunks. (If desired, save enough dough to make 12 little cookie dough balls as garnish on top!) With the mixer running on medium speed, add the cookie dough chunks. Mix just until incorporated—some chunks can remain.

Once the cupcakes are completely cool, top with frosting. You can spread the frosting 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.) Decorate with chocolate chips, sprinkles, or any leftover cookie dough, if desired.

Leftover cupcakes keep well covered tightly in the refrigerator for 3 days.