Preheat the oven to 175°Celsius.
Line baking trays with parchment paper. Set aside.
In a medium-sized bowl, cream butter and sugar until creamy. Scrape the sides of the bowl.
Add the egg yolk, followed by vanilla extract. Beat until the traces of the egg yolk are no longer visible and the vanilla is well combined. Scrape the sides of the bowl.
Combine cornstarch, all purpose flour, baking powder and salt into a bowl and sift these dry ingredients into the creamed mixture.
Mix until well incorporated. Scrape the sides of the bowl.
Add the cornflakes and mix into the dough on low speed. This will help to crush the cornflakes into the cookie dough (see Note 1 for mixing by hand).
Shape the cookies by pressing the dough into a '1 tablespoon' measuring spoon. Scoop the pressed dough out and form into ball and flatten it lightly with your fingers.
Pour sprinkles into a small bowl. Dip the top of the flattened cookies into the sprinkles. Press slightly to attach the sprinkles to the cookies.
Turn the cookies over and place them on the baking tray.
Bake for 12 minutes until the sides turn light brown. Remove cookies from the oven and cool them over a wire rack.
Store cookies in an airtight container to retain their crispiness.