In a large bowl, using a hand mixer, beat the butter, brown sugar, and granulated sugar together on high until light and fluffy, about 3 minutes.
1 cup unsalted butter, 1 cup brown sugar, ½ cup granulated sugar
Beat in the eggs and vanilla extract until combined.
2 large eggs, 1 tablespoon pure vanilla extract
In a separate bowl, whisk the flour, cornstarch, baking powder, and salt together.
3 cups all-purpose flour, 1 tablespoon cornstarch, 1 teaspoon baking powder, ½ teaspoon kosher salt
Add the dry ingredients to the wet ingredients and beat on low just until mixed. Be careful not to beat the dough too much or the cookies will lose that chewy texture.
Stir in 1 cup of the chocolate chips. Set aside the remaining ¼ cup chocolate chips for now.
1¼ cups semisweet chocolate chips
Using a 1½-tablespoon-sized cookie scoop, scoop out 48 cookie dough balls. Each cookie dough ball will be about 26 grams.
Take two cookie dough balls and flatten them with your hands.
Take about 1 tablespoon of soft brownie and squish it into a tight ball using your hands.
1/2 pound prepared brownies
Place that brownie ball on top of one flattened cookie dough ball and place the other flattened cookie dough ball on top. Pinch the sides of the two flattened cookie dough balls together and roll this around in your hands until the brownie is completely covered by cookie dough. Repeat steps with the rest of the cookie dough. You’ll end up with 24 stuffed cookies in total.
Once all cookie dough balls are stuffed with brownies, use the remaining ¼ cup of chocolate chips to press chocolate chips into the tops of each cookie. The cookies can be close together on a lined baking sheet now – but not touching – so you just have one tray to place into the freeze for the next step.
Place cookies in freezer for 10 minutes before baking. This will help prevent the cookies from spreading during baking.
While the cookies chill, preheat oven to 350°F. Line a second baking sheet with parchment.
Take some cookies out of the freezer and place them on the prepared baking sheet at least 2 inches apart from each other.
Bake each tray of cookies for 18 minutes, or until the bottoms of the cookies slightly brown. The tops may not look completely set yet but that’s okay.
After removing the baking tray from the oven, leave the cookies on the hot baking sheet for 15 minutes before transferring to a wire rack to cool completely. During those 15 minutes on the hot baking tray outside of the oven, the cookies will finish baking.