In a medium bowl, combine the flour, baking soda, cornstarch, and salt. Set aside.

Either using a stand or a hand-held mixer cream together the butter and sugar.

While still mixing on medium speed, add in the egg and egg yolk, then add in the vanilla extract.

Combine the dry ingredients into the wet ingredients on medium-low speed.

Fold in the festive holiday sprinkles with a plastic spatula or a spoon.

Cover the mixing bowl with plastic wrap, then let the cookie dough chill in the refrigerator for 2 hours.

Preheat oven to 350 degrees and line a cookie sheet with parchment paper or a silicon baking mat.

Pour ½ cup sprinkles into a small bowl and set aside.

Spoon cookie dough and roll into balls, then roll into the sprinkles and place on cookie sheet.

Bake for 9-10 minutes. Let the cookies cool for 1-2 minutes before transferring to a cooling rack.

Repeat until all cookies have been baked.