Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan with butter or cooking spray.

In a large mixing bowl, cream together the butter, brown sugar, and granulated sugar until light and fluffy.

Add the eggs, sour cream, and vanilla extract to the creamed mixture and mix until just combined.

In a separate bowl, whisk together the flour, baking powder, baking soda, ground cinnamon, and salt.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.

Spread the batter evenly into the prepared baking pan.

In a small bowl, mix the granulated sugar and ground cinnamon together, then sprinkle it over the batter.

Bake in the preheated oven for 25-30 minutes or until golden brown.

Cool the bars completely in the pan on a wire rack.

Make the frosting by whisking together the powdered sugar, milk, and optional vanilla.

Spread the frosting over the cooled bars, cut into squares, and serve.