Melt butter in the microwave for 30 seconds and let rest on counter for 10-15 minutes.
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.
Once the dough is ready let it rest on the counter for 10 minutes before rulling it into balls.
Begin making the dough into balls (about 2 tablespoons of dough for each ball) and let them rest again in a covered container for 30 minutes in the refrigerator.
peheat oven to 425 degrres
Place about 2 inches apart on baking sheets and bake for 9-10 minutes.
Let me rest on the counter for 5 minutes before moving to a cooling rack.
