Preheat your oven to 350°F. Line 2 large baking sheets with parchment paper, and set aside. In a medium sized bowl, combine flour, cornstarch, baking powder, baking soda, and salt. Whisk together, and set aside.
In a large bowl, combine butter and sugar. Using an electric mixer fitted with the beater attachment or a wooden spoon by hand, cream together butter and sugar until light and fluffy (about 2 minutes on medium speed or 5 minutes by hand).
Add the egg and vanilla to the butter mixture, and mix together until combined.
Add your dry ingredients to the butter mix, and stir together until combined. Fold the jimmies sprinkles into the dough until well distributed.
Portion the dough into balls that are about 2 tablespoons large (you can use a tablespoon and roll the dough between your hands or a cookie scoop). Arrange the portions of dough onto your parchment lined baking sheets, leaving about 1 inch of space between each piece of dough.
Bake the cookies in the center of your preheated 350°F oven for about 7 to 10 minutes, or until the cookies are puffed up and no longer shiny. These cookies will be quite pale in color if you want them chewy. For crispy cookies, bake until the edges have just started to brown.
Cool the cookies for about 5 minutes on the baking tray, then transfer onto a wire rack to finish cooling completely to room temperature.