Preheat oven to 350F. Generously butter the sides and bottom of a 10" cast iron skillet and set aside.
In a KitchenAid or using a handmixer, beat butter until creamy.
Add in sugars and beat until light and fluffy.
Stir in eggs, one at a time, and then vanilla extract. Make sure to scrape the sides and bottom of the bowl periodically so the ingredients are mixed well.
In a separate bowl, whisk together flour, cornstarch, baking powder, and salt.
Gradually stir flour mixture into the butter mixture, until the ingredients are completely combined.
With your mixer on low-speed, stir in the sprinkles.
Pour out into 10" skillet and bake on 350F for 30-35 minutes (the edges should just be turning golden brown).
Allow to cool for 10 minutes or so. Top with ice cream, chocolate sauce, and sprinkles (if desired) and dig in! Be careful, though, the skillet will be very hot!
You may also allow the blondie to cool completely, and then slice into wedges and enjoy as is.