Preheat oven to 350 degrees F. Line a 9x13" pan with tin foil and spray with non stick spray.
In a large bowl, combine melted butter and brown sugar and whisk until smooth. Add egg and whisk until combined. Add vanilla, salt, and flour and stir just until combined. Stir in sprinkles.
Spread into prepared pan and bake for 20-22 minutes, until a toothpick comes out clean in the center. Set aside to cool to room temperature.
FrostingIn a medium bowl, beat butter with an electric mixer until smooth. Add sugar, vanilla and 2 tablespoons milk and beat until smooth. If desired, add a little more milk or sugar to reach desired consistency. Stir in sprinkles and spread over cooled bars.