To the bowl of a stand mixer fitted with the paddle attachment, combine the butter, cream cheese, sugars, egg, vanilla, and beat on medium-high speed until well-creamed, light and fluffy, about 5 minutes (or use an electric hand mixer and beat for at least 7 minutes).

Stop, scrape down the sides of the bowl, and add the flour, cornstarch, baking soda, optional salt, and mix until just combined, about 1 minute.

Add chocolate chips and chunks, and beat momentarily to incorporate, or fold in by hand.

Using a medium 2-inch cookie scoop, form heaping mounds (I made 28). Place mounds on a large plate, flatten mounds slightly with your palm, cover with plastic wrap, and refrigerate for at least 2 hours, or up to 5 days, before baking.

Preheat oven to 350F, line a baking sheet with a Silpat Non-Stick Baking Mat or spray with cooking spray and place mounds on baking sheet, spaced at least 2 inches apart (I bake 8 cookies per sheet).

Bake for 8 to 9 minutes, or until edges have set and tops are just beginning to set. Do not bake longer than 10 minutes.

Allow cookies to cool on the baking sheet for 5 minutes before removing and transferring to a rack to finish cooling.

Cookies will keep airtight at room temperature for up to 1 week, or in the freezer for up to 3 months.