Preheat your oven to 350°F (175°C) to ensure even baking.
In a medium bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and salt.
In a large mixing bowl, cream together the unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes.
Beat in the egg and vanilla extract until well combined.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Gently fold in the rainbow sprinkles until evenly distributed.
Using a cookie scoop, portion out the dough onto a lined baking sheet.
Bake for about 10-12 minutes, or until the edges are lightly golden.
Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a cooling rack.
Prepare the frosting by beating together the unsalted butter and dry vanilla cake mix until creamy. Gradually add the powdered sugar and vanilla extract.
Once the cookies are cool, spread a generous layer of frosting on top of each cookie.
