2cupssemisweet chocolate chunks or chocolate chipsdivided (340g)
Preheat oven to 350F (175C) and line cookie sheets with parchment paper. Set aside.
In a stand mixer (or using an electric hand mixer and a large bowl), beat together butter and cream cheese until creamed.
Add sugars and beat on medium-high speed for 2 minutes, until light and fluffy. Pause mid-way through to scrape down the sides of the bowl.
Add egg and beat until combined, and stir in vanilla extract.
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking soda, cream of tartar, and salt.
With mixer on low speed, gradually add flour mixture to your wet ingredients until completely combined.
Using a spatula or with your mixer on low-speed, stir in 1 ½ cups of the chocolate chunks/chips, and pause to scrape down the sides of the bowl so that chocolate is well incorporated.
Scoop dough by 1 ½ Tablespoon-sized balls and gently roll between your palms for a smooth ball. Place on cookie sheet, spacing at least 2" apart, and bake on 350F (175C) for 8-10 minutes. Cookies may still seem soft in the centers but will continue to bake on the cookie sheet as they cool.
Within one minute of removing from the oven, gently press remaining chocolate chunks/chips into the tops of each cookie. Allow cookies to cool on baking sheet for at least 10 minutes.