Combine the butter and sugar until smooth. Add the vanilla and egg.
Mix in the flour, cornstarch and baking soda then add in the sprinkles.
Form the dough into even balls and press them down slightly on a baking sheet. Cover and place the baking sheet in the fridge for at least an hour.
Preheat the oven to 350 and bake for 10 minutes then leave them on the sheet for about 5 minutes before transferring to a cooling rack. Store in an airtight container.
Notes
½ cup of butter weighs 113.4 grams. This is handy to know if you're using a block with no markers left on the package.
Recipe by Friday is Cake Night at https://www.fridaycakenight.com/2016/01/valentine-sprinkle-cookies/