To the bowl of a stand mixer combine the butter, sugars, egg, vanilla, and beat on medium-high speed until creamed and well combined, about 4 minutes.
Stop, scrape down the sides of the bowl if needed, and add the flour, cornstarch, baking soda, optional salt, and beat on low speed until just combined, about 1 minute.
Add the pretzels, chocolate chips, and beat on low speed until just combined or pretzels are as small as desired, about 1 minute.
Using a 1/4-cup measuring cup, or your hands, form 10 equal-sized mounds of dough (they look fairly large because the pretzels are bulky), roll into balls, and flatten slightly.
Preheat oven to 350 degrees, line a baking sheet with parchment paper. Place dough mounds on baking sheet, spaced at least 2 inches apart and bake for about 10 to 12 minutes, or until edges have set and tops are just set, even if slightly undercooked, pale, and glossy in the center; don't over-bake them. Cookies firm up as they cool. Allow cookies to cool on baking sheet for about 10 minutes before serving.