Preheat oven to 350 degrees F. and line a 9 x 13 inch baking dish with parchment paper.
Place butter and sugars into a large mixing bowl or stand mixer. Beat until well combined. Add egg, vanilla and sour cream mixing until well combined. Stir in flour, baking powder and salt, stirring until thick dough forms. Transfer dough into prepared baking dish, pressing evenly in bottom of dish. Bake for 18-20 minutes. Remove and let cool completely.
To prepare frosting, place cream cheese and butter into a mixing bowl or stand mixer with whisk attachment. Beat cream cheese and butter until creamy and smooth. Slowly beat in powdered sugar until desired consistency. Add a drop or two of food coloring if desired. Spread over cooled bars. Top with sprinkles of your choice if desired. Cut bars into squares and serve room temperature.