Heat the oven to 350 degrees F. and line a 9x13 inch pan with parchment paper or tin foil and spray with non-stick spray.
In a large bowl, whisk the melted butter and brown sugar until smooth. Add the eggs and stir until combined. Add the vanilla, salt, and flour and stir until just combined. Stir in the sprinkles.
Spread into the prepared pan and bake at 350 degrees F for 20 to 22 minutes, or until a toothpick comes out clean in the center. Set aside to cool to room temperature.
FROSTING
In a medium bowl, beat the butter with an electric mixer until smooth. Add the 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 the sprinkles and spread over the cooled bars.