In the bowl of a stand mixer {fitted with paddle attachment} combine butter, sugar, eggs and vanilla extract.  Beat on medium speed until creamed {it should be light and fluffy}.  This will take about 5 minutes but do not skip this part.  Scrape down sides of bowl as necessary.  

Add flour, cornstarch, baking soda and salt and mix until just combined.  Add sprinkles and fold in by hand.

Using a medium cookie scoop {or 2-3 tablespoons of dough} form mounds.  Place on a pan lined with parchment or wax paper, cover with plastic wrap and refrigerate for at least 2 hours and up to 5 days.

When ready to bake preheat oven to 350 degrees.  Line cookie sheets with parchment or spray with cooking spray.  Place mounds on sheets, being sure to space them about 2 inches apart.  Bake for 8-9 minutes, or until edges are set and the middle of the cookie is just beginning to set.  They will look under baked but let them cool on cookie sheet for 5-7 minutes before moving to a cooling rack and they will firm up as they cool.

Store cookies in an airtight container at room temp for up to a week or freeze for up to 3 months.