Preheat oven to 350. Line a 9x13 inch pan with aluminum foil or parchment paper leaving an overhang on both sides for easy removal.
Spray foil with non-stick cooking spray.
In a large bowl use a hand mixer, or stand mixer with paddle attachment and beat butter together with sugar and vanilla on medium speed until fluffy (2-3 minutes) then, beat in the eggs.
In a medium bowl, whisk together the flour, baking powder, and salt.
With your mixer running slowly add dry ingredients into the wet ingredients until completely combined.
When dough is combined, turn of mixer and fold 1/2 cup of sprinkles into batter.
Spread and press dough evenly into pan.
Bake for 25-30 minutes, just until very very lightly golden and edges are set. Careful not to over bake.
Allow bars to cool completely before frosting and slicing.
For the frosting:
In a medium bowl using your hand mixer, or in the bowl of a stand mixer beat butter for 1-2 minutes until smooth and creamy.
With mixer on low, add powdered sugar, cream or milk, vanilla and salt.
Turn mixer to high and continue to beat for 2-3 more minutes.