For the blondies: Preheat the oven to 350 degrees F. Prepare an 8-inch square baking pan with parchment and cooking spray, allowing the parchment to hang over the sides.
In a medium bowl, whisk together the brown sugar and butter. Whisk in the vanilla and eggs. Using a rubber spatula, fold in the flour, baking powder and salt. Lastly, fold in the sprinkles.
Pour the batter into the prepared baking pan and spread it to even out the surface. Bake until the blondies are golden brown, 32 to 35 minutes. Set aside to cool completely before icing, about 1 hour.
For the confetti icing: In a large bowl, mix the powdered sugar, butter, vanilla and salt with an electric mixer until just combined. Add the milk and whip until the icing is light and fluffy, 2 to 3 minutes. Fold in the rainbow sprinkles.
Spread the icing on the cooled blondies in a thick even layer. Sprinkle the top with as many sprinkles as your heart desires. Refrigerate the baking pan until the icing is firm, at least 1 hour.
Carefully lift the blondies out of the pan and cut into 2-inch squares.