Start by melting the butter. Place 1/2 cup of unsalted butter that's been cut into tablespoon-sized pieces in a microwave-safe bowl and cover it with a paper towel (trust me, there is nothing worse than butter exploding all over your microwave!!!). Heat in 30-second intervals at half power until it's fully melted, then set aside. I like to let my butter sit for about 10 minutes once it's melted to cool down a bit. This prevents the butter from melting the mini chocolate chips and helps the cookies spread less while they bake.
While the butter cools, preheat the oven to 350°F/175°C and line two 9x13-inch baking sheets with silicone mats or parchment paper. If you only have one tray or smaller baking sheets, that's fine! The cookie dough can sit at room temperature for several hours as the mini cookies bake and cool.
Whisk together 1 1/3 cups flour, 1 1/2 tsp cornstarch, 1/2 tsp baking soda, and 1/2 tsp salt in a large bowl. Set aside.
Pour the melted and cooled butter, 2/3 cup packed brown sugar, and 1/4 cup granulated sugar into a large bowl. Whisk the ingredients together by hand or with a hand mixer for about a minute, until the mixture is smooth, and no lumps remain. If you notice any lumps of brown sugar, use a fork to break them up. The mixture should become lighter in color as it's whisked.
Whisk in 1 large egg and 1 tsp vanilla extract. Stir until the egg is fully incorporated.
Pour the dry ingredients into the butter/sugar mixture and mix together with a rubber spatula until no visible streaks of flour remain. Scrape the bowl's sides and bottom as needed to ensure everything gets properly mixed together. The dough will be soft/sticky, but that's how it's supposed to be. We need it to be quite soft to be able to pipe the cookies in step 8!
Fold in 1 cup of mini chocolate chips or chopped-up chocolate chips of your choice. Scoop the dough into a large piping bag (I used an 18" bag) or freezer Ziploc bag and seal the top with a rubber band or clip. Cut an opening at the base of the bag that is about 1 inch or 2.5 cm in diameter.
Pipe nickel-sized dollops (about 2 cm in diameter) of cookie dough onto the prepared baking sheets, spacing them about 1 1/2 inches or 4 cm apart. I was able to fit about 35 cookies on each 9x13" baking tray. You should use about half of the dough to pipe the first two trays of cookies.
Bake one tray of cookies at a time for 6-8 minutes. Bake time can vary a lot based on your oven, the type of pan you use, if you use silicone mats or parchment paper, how many trays you bake at a time, and whether you want chewy or crispy cookies. The sweet spot in my oven is baking one tray at a time for 7 minutes. The cookies are ready when the edges just start to brown but the centers still look slightly under-baked. They will continue to bake on the sheet and firm up once removed from the oven, so it's always best to err on the side of under-baked!
Wait for the first pan to bake and cool, then see if you're happy with the doneness and adjust the bake time if needed for the following pans. Note: If you are short on time and need to bake two trays at once, bake them for 8-9 minutes and swap the shelves that the pans are on halfway through to help them bake evenly. Bake time is slightly longer because heat will escape the oven as you open it.
Once the cookies are done baking, place the cookie sheet on a wire rack and let the cookies cool fully on the pan. I like to press a few extra chocolate chips on top of each cookie while they're still warm and add a garnish of flaky sea salt. Remove the cooled cookies from the baking sheet, then pipe and bake another batch of cookie dough. Repeat until you've used up all the cookie dough.
Enjoy warm with a glass of milk, or store fully cooled cookies in an airtight container at room temperature for up to 3 days.