Weigh or measure the dark brown and white sugar into a bowl and set aside. In another bowl, weigh or measure the flour, baking powder, salt, and cornstarch, and run a whisk through to blend.
With a stand or handheld mixer, I cream the butter for a minute before adding the sugars. Start on low speed, then increase until you reach a medium speed. Continue at medium speed for 2 to 3 minutes. Stop and scrape down the sides, and add the vanilla extract. Crack the egg into a small bowl to ensure no eggshells, then add the egg to the mixing bowl. Blend on medium speed until the egg is incorporated.
Turn the mixer to a low speed and slowly add the flour mixture. After adding the flour, I stopped the mixer and scraped down the sides and across the bottom. Turn the mixer on for another 5 seconds to incorporate the last flour mixture into the cookie dough. Stop and add the chunky chocolate pieces, dried cherries, and pecans, and turn the mixer on for a few seconds. The batter is thick, so you may have to mix a little by hand.
Preheat over to 350°F (175°C or 180°C) and line 2 cookie sheet pans with parchment paper. To create uniform-size cookies, I use a medium cookie scooper. I got 12 cookie mounds to a pan. You can always make these a drop cookie using a spoonful of cookie dough. I stick the whole cookie sheet pan with the unbaked cookies on it into the refrigerator for 10 to 15 minutes before popping them into the oven.
Bake for 10 to 12 minutes, but start checking after 10 minutes. You do not want to see dark brown around the bottom edges of the cookie. They should be light to medium brown on the bottom, mainly due to the dark brown sugar. After pulling the cookies out of the oven, I leave them on the sheet pan for a minute before moving them to a cooling rack. They firm up as they cool.