To the bowl of a stand mixer fitted with the paddle attachment, combine the butter, cream cheese (measure it by smooshing it into a ¼-cup measure), 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. Do not bake with warm dough because cookies will spread and bake thinner and flatter.

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, even if slightly undercooked, pale and glossy in the center. Do not bake longer than 10 minutes as cookies will firm up as they cool.

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. Alternatively, unbaked cookie dough can be stored airtight in the refrigerator for up to 5 days, so consider baking only as many cookies as desired and save the remaining dough to be baked in the future when desired.