Preheat oven to 350 degrees F
In a large mixing bowl, add your softened butter. Cream your butter for about 1 minute.
Add your sugar to your butter and combine. Slowly add in your eggs one by one. Add in your vanilla.
In a separate bowl, combine your Flour, baking powder, baking soda and salt. Mix well.
Add in your dry ingredients slowly into your wet. Don't mix too long. One minute or so.
Spray an 8x8 (9x9, 9x13 will work too) with non stick spray. Add your batter. Bake for 20-25 minutes, using the toothpick method to test doneness.
While your bars are baking, make your frosting. Add your butter to the mixing bowl and cream. Add in your powdered sugar one cup at a time along with your vanilla and salt.
Have fun with your frosting! Add cream little by little until your desired doneness. You want it to be a little stiff and not runny.
Obviously, finish with sprinkles.