Preheat oven to 350
Melt 2 sticks of butter. While butter is cooling, prepare a 9x 13 baking dish by lining it with foil, leaving extra to hang over the sides of the dish.
Coat foil with cooking spray
Pour cooled, but not cold, melted butter into mixing bowl.
Mix in sugar, eggs, and vanilla.
Add flour and salt.
Add sprinkles. Mix until combined.
Pour mixture into prepared dish.
Bake until the edges are set, but the middle is soft, approximately 25 minutes.
Let cool. Pull bar out of pan by pulling on foil, and flip onto large plate or cookie sheet.
Frost cookie and top with additional sprinkles! Cut into squares and serve.