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 pieced of dough (40g), roll into balls and top with some more sprinkles.
Place about 2 inches apart on baking sheets lines 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.
You don’t want to add your sprinkles too early into your dough. Add the sprinkles in at the end and mix until it is just combined.
Do not skip chilling the cookie dough. Chilling the dough prevents the cookies from spreading out too quickly once they're in the oven and you won’t get the round fluffy cookies look like I have here.
Make sure your butter and egg are at room temperature so everything is mixed and baked evenly.
Even though it’s only 1 tablespoon, the cornstarch is essential for soft cookies so don’t skip it.
Use baking parchment or a silicone mat so your cookies don’t stick to your baking tray or cookie sheet.
While the centers may look soft when you pull the cookies out, they will firm up as it cools. If you leave them in the oven until the middles are firm, your cookies will overbake and be crunchy once they’ve come to room temperature.
You can leave the cookies to chill in the fridge overnight if you want to make these ahead of time.
Roll your dough into balls before chilling it as your dough will not be as malleable after it has chilled.