Preheat your oven to 375F. Line your cookie sheets with parchment paper or silicone baking mats.
Combine your flour, baking power, and cornstarch in a bowl and set aside. **If using cocoa powder, it can be added to your dry ingredients as pictured**
Place your cool unsalted butter, both sugars, and salt into your stand mixer. With the paddle attachment, mix everything together on low until it's incorporated. We are NOT creaming everything together! If you've ever made any of my sugar cookies, we're starting these the same as those.
Once it's all incorporated, add your eggs and your vanilla or other chosen extract. Mix until incorporated.
This is what it will look like before you add your dry ingredients.
Scrape down the bottom and sides of your mixer. Add your flour and baking soda mixture. Mix on low until the flour is almost incorporated. **I have cocoa powder in mine, but it's optional**
This is what it will look like before you add your chocolate chips.
Add your chocolate chips, crushed cookies, nuts, or crushed candies now and mix on low just until all the add-ins are mixed in.
Scrape down the bottom and sides of your bowl just to make sure everything is incorporated.
Using a small cookie scoop (about 1 1/2 tablespoons), scoop dough onto your cookie sheets and bake in your preheated oven for 6-8 minutes. If you like a crunchy edge with a soft middle, 8 minutes is just about perfect. If you want softness all the way around and no browning then stick with 6-7 minutes.
Leave them on the pan for a few minutes (no more than 5) after removing from the oven and they will continue to bake a bit more.
After a few minutes on the pan, move them to a cooling rack to cool completely.