Combine the flour, salt, cornstarch, and baking soda in a bowl then whisk together and set aside. In a mixer fitted with the paddle attachment cream the butter and sugar. Mix on high until light and fluffy. Mix in the egg and vanilla until well combined.
Add the dry mixture to the butter mixture and mix on low until almost combined. Add the sprinkles and mix on low until just combined. Using a medium cookie scoop (about 2 Tablespoon) scoop out cookie dough onto a parchment lined baking sheet. Pat extra jimmies on top of each cookie. Place baking sheet in fridge for one hour. Preheat oven to 350. Place cold baking sheet in preheated oven and bake 11-12 minutes. Bake just until edges are set. Centers will be slightly soft, but will continue to bake while sitting on cookie sheet for 5 minutes. Then remove to a wire rack to cool completely.