Preheat the oven to 350°F. Line a 9x13 pan with foil and coat with nonstick spray. Set aside.

In a large bowl whisk together the butter and sugar. Add in the vanilla, eggs and salt and whisk until smooth. Using a rubber spatula stir in the flour until just combined. Finally stir in the sprinkles.

Spread the batter into the prepared pan.

Bake for 25-30 minutes until the edges are set. The bars might seen underdone, but allow them to cool completely.

In the bowl of the stand mixer beat the butter for 1-2 minutes until it's smooth. Turn the mixer to low and slowly add in the powdered sugar. Next add in 2 tablespoons of milk and mix on medium speed until it's creamy, scraping the sides as necessary. If the frosting is too stiff add in 1 more tablespoon of milk until the desired consistency is reached. Finally add in 1-2 drops of blue fool coloring and mix until a pale blue color is reached and evenly mixed.

Pipe or spread the frosting on top of the bars.

Garnish with more sprinkles if desired.