In a medium bowl, whisk together the all-purpose flour, cornstarch, and baking powder until well combined. Set aside.
In a large mixing bowl, add the room temperature unsalted butter and cream cheese. Beat with an electric mixer on medium speed until smooth and creamy with no lumps remaining.
Add the egg, milk, and vanilla extract to the butter–cream cheese mixture. Mix on medium speed until fully combined.
Gradually add the powdered sugar, starting on low speed so it doesn’t puff out of the bowl, then increasing to medium speed. Beat until the mixture becomes light and fluffy.
Add the flour mixture to the wet mixture. Mix on low speed just until the dough comes together and looks smooth and thick. Do not overmix. Fold in the mini chocolate chips with a spatula or spoon until evenly distributed throughout the dough.
Cover the bowl and refrigerate the dough for 1 hour, or until firm enough to handle.
Towards the end of the chilling time, preheat the oven to 375°F (190°C). Line two baking sheets with parchment paper.
Remove the dough from the refrigerator. Roll the dough into 24–26 balls, making them as even in size as possible. Place the balls on the prepared baking sheets, about 1.5 inches apart.
Gently flatten each ball with your hand or the bottom of a glass to form thick disks. If desired, press a few extra mini chocolate chips on top of each cookie.
Bake for about 10 minutes, or until the edges are just set and the centers still look slightly soft. Do not overbake.
Remove the baking sheets from the oven and let the cookies cool on the sheets for a few minutes. Transfer the cookies to a wire rack to cool completely. Serve and enjoy.