Combine the flour, salt, cornstarch, and baking soda in a bowl then whisk together and set aside.

Cream the butter and sugar in the bowl of your stand mixer fitted with a paddle attachment, mix on high until light and fluffy. Mix in the egg and vanilla then scrape the bowl down and mix once more until well combined.

Add the dry mixture to the butter mixture and mix on low until almost combined. Scrape down the bowl then add the sprinkles and mix in low until just combined.

Use your spatula to fold in any extra flour hanging about then portion out two tablespoon-sized pieces of dough (40g), roll into balls and top with some more sprinkles.

Place about 2 inches apart on baking sheets lined with silicone mats or parchment paper then chill the dough balls for at least an hour.

Towards the end of your chill time preheat oven to 350F, then bake for about 11 minutes or until the edges are set and cookies are puffed. They will continue baking after removed from oven so allow to cool on the baking sheets for about 5 minutes.