Preheat the oven to 350 degrees Fahrenheit. Line two cookie sheets with parchment paper, or silicone liners, and set it aside.
Add the butter and sugars to a large mixing bowl (or the bowl of a stand mixer) and beat them together for about 2 minutes, until the mixture is light and creamy. 1/2 cup Unsalted Butter, 1/2 cup Granulated Sugar, 1/4 cup Brown Sugar
Add in the egg, and vanilla extract and stir them into the mixture until combined. 1 large Egg, 1/2 tsp Vanilla Extract
Add in the flour, baking powder, baking soda, and salt and stir it all together until you have a nice soft cookie dough. Scrape the bottom and sides of the bowl as needed. 1 1/2 cups + 2 TBS all purpose flour, 1/2 TBS Cornstarch, 1/2 tsp Baking Soda, 1/2 tsp salt
Fold in the sprinkles until evenly mixed into the cookie dough. 1/3 cup Jimmie Sprinkles
Scoop the cookie dough into 1 teaspoon portions and roll them into balls.
Place 3 to 4 extra Tablespoons of sprinkles into a small bowl, and roll the cookie dough balls into the sprinkles to coat the outsides.
Place the cookie dough balls at least 1 inch apart on the prepared baking sheets. Bake the cookies for 5 to 7 minutes, or until the tops are set (and no longer shiny) and the edges are very lightly golden.
Remove from the oven and let the cookies cool for 10 minutes on the cookie sheet, then transfer them to a cooling rack to cool completely.