Preheat oven to 350°F and line 8x8 baking pan with parchment paper.
In a medium mixing bowl, whisk together flour, corn starch, baking soda, and salt.
In a large mixing bowl, stir together butter, sugar, and vanilla.
Mix egg into wet ingredients. (If butter is still quite warm from melting, be sure to mix egg in rapidly so that it doesn't become a scrambled egg in the batter.)
Gradually stir dry ingredients into wet ingredients until almost completely blended together.
Add 1/3 cup sprinkles to cookie dough and mix until evenly distributed. Dry ingredients should also now be fully incorporated.
Press dough into prepared pan. Add remaining tablespoon of sprinkles on top if you choose using (no need to press these into the dough).
Bake 30 minutes or until slightly golden on top and a test toothpick comes out clean.
Let cool completely before cutting into 16 squares.