Why do we beat in the baking powder into the butter? Normally you would sift it in with your other dry ingredients..

The only reason I can think of would be so it would increase the amount of air that could potentially be whipped into and retained by the sugar mixture and eggs. It's such a small amount that it isn't going to be able to provide much in the way of leavening power otherwise. Baking soda also changes the pH of your mixture, and acts along with the fat and egg yolks in hindering gluten development. The sour cream will actually encourage gluten formation, though it makes for a tender crumb. So it may be the odd out sequence of mixing helps balance things out in the final product. I've seen other recipes call for this as well, usually for a quick bread- which this is cousin to. That's the only reason I can think of to explain it.