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

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

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

Using a medium 2-inch cookie scoop, form heaping mounds (this recipe makes approximately 28 cookies). Place mounds on a large plate, flatten them 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 as cookies will spread and bake thinner and flatter.

Preheat oven to 350°F (175°C). Line a baking sheet with a Silpat non-stick baking mat or spray with cooking spray. Place chilled cookie mounds on the baking sheet, spaced at least 2 inches apart (bake approximately 8 cookies per sheet).

Bake for 8 to 9 minutes, or until the edges have set and the tops are just beginning to set, even if slightly undercooked, pale, and glossy in the center. Do not bake longer than 10 minutes, as the cookies will firm up as they cool.

Allow the cookies to cool on the baking sheet for 5 minutes before removing and transferring them to a wire 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. Consider baking only as many cookies as desired and save the remaining dough to be baked later.