Preheat the oven to 375 degrees Fahrenheit. Line two cookie sheets, with parchment paper, or a silicone liner.
Cut your butter into slices. Add the slices butter into a large mixing bowl (a stand mixer with a paddle attachment works great if you have it), start to beat the butter, for about 1 minute.
Add the brown sugar, and granulated sugar and beat for 2 minutes, until the mixture is smooth and creamy.
Add in the eggs and vanilla extract, and stir until just combined.
Dump in the flour (I recommend doing the scoop and level method for measuring it out or you may end up with too much), cornstarch, baking soda, and salt. Stir it together till you have a nice soft dough, scrape the bottom of the bowl as needed.
Add in the chocolate chips and sprinkles and fold them evenly into the dough.
Measure out the dough to be about 4 oz each (12 even sized portions). Roughly push the dough into a rounded shape, but don't totally roll it like a ball.
Top the cookie dough ball with a few chocolate chips and sprinkles.
Place 4-6 cookie balls onto each prepared cookie sheet.
Bake in the preheated oven for about 10-12 minutes, until the tops of the cookies are set and golden. I usually pull mine out at 10 minutes, but adjust to your liking depending on how gooey you want the middles.
Remove from the oven and let the cookies cool on the pan for about 10 minutes.
Transfer the cookies to a cooling rack to cool completely, or enjoy them warm.