Preheat oven to 350 degrees Fahrenheit and line a 9"x13" pan with parchment paper so it hangs over the edges.

In the bowl of an electric mixer, cream together softened butter and sugar until fluffy.

Add vanilla, eggs and sour cream and mix completely, scraping down the sides in between.

In a separate bowl, whisk together flour, baking powder and salt. Add flour mixture to wet mixture and mix JUST until incorporated. Don't over-mix.

Spread batter evenly out into parchment-lined 9"x13" pan.

Bake for approximately 18-20 minutes, or just until the bars are set. You don't want them to be jiggly when you pull them out to check on them. You don't want to start seeing any brown along the edges or on top. Mine were still light on top when they were done. Testing with a toothpick isn't ideal for cookie bars, but it can help. You may still see moist crumbs, but you don't want to see runny batter.

Allow them to cool completely, then prepare the frosting.

For the frosting, cream together softened butter, powdered sugar, vanilla and just enough milk to thin it, until fluffy. Mix it at a high speed at least for a few minutes and up to five minutes for the fluffiest, most divine frosting! Frost cooled bars and add sprinkles, if desired. Slice into squares and serve.