1. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cornstarch. Set aside.
3. In a separate large bowl, beat the softened butter and granulated sugar together until creamy and smooth.
4. Add the egg and vanilla extract, and beat until well combined.
5. Gradually add the dry ingredients to the butter mixture, mixing until just combined.
6. Scoop tablespoon-sized portions of dough and roll them into balls. Place them on the prepared baking sheet, spacing about 2 inches apart.
7. Gently flatten each dough ball with the bottom of a glass or your hands to form thick cookie rounds.
8. Bake for 10–12 minutes, or until the edges are just starting to turn golden. The centers should remain soft.
9. Let the cookies cool on the baking sheet for a few minutes, then transfer them to a wire rack to cool completely.

For the Frosting:

1. In a bowl, beat the softened butter until creamy.
2. Gradually add the powdered sugar, beating until smooth.
3. Add the vanilla extract, corn syrup, and milk, and beat until fluffy and spreadable. Add more milk if needed for consistency.
4. Once the cookies have cooled, spread the frosting generously on top of each cookie.
5. If desired, add food coloring to the frosting and decorate with sprinkles.