Preheat the oven to 350℉. Grease a 9 x 13 inch baking pan (or spray with non-stick cooking spray).
In a large bowl, beat together the butter, brown sugar, and sugar. Add in the egg, vanilla, and sour cream and mix until completely incorporated.
Add in the flour, cocoa powder, baking powder, baking soda, and salt and mix well.
Stir together the flour, cocoa powder, baking powder, and salt. Press the dough into the prepared pan (I had to use my hands to press the thick dough all the way to the edges of the pan) and bake for 13-14 minutes (be sure not to over bake or they will become dry).
While waiting for the bars to cool, make your frosting by mixing all ingredients together in a bowl until smooth (I used a hand mixer). Add more powdered sugar or milk until you reach desired consistency and spread on top of cooled bars.
If desired, top frosting with sprinkles.
Notes
If you have any of these bars leftover we recommend covering them with a lid, or placing them in an airtight container. They will be good on the shelf or counter for about two to three days before they begin to get dry.I also love to freeze them, or place them in the refrigerator. I also love eating them cold. I highly recommend trying them cold.