Preheat oven to 325 degrees, spray a loaf pan with cooking spray and set aside. Using a hand held or stand mixer, mix the butter and sugar together until fluffy, add in eggs one at a time. Mix together the milk, vanilla and sour cream in a measuring cup. Mix together the dry ingredients except the cocoa powder. Slowly add some of the milk mixture and then the dry mixture until all is incorporated. Stop the mixer and finish by mixing by hand with a rubber spatula. Now, add in the cocoa powder. Mix again. Take a spatula and make sure you dig down to incorporate any leftover flour mixture. Pour into the prepared loaf pan. Spread out evenly. Bake for about 42 to 50 minutes. Check after 42 minutes. The cake should spring back when touched lightly on top and a toothpick should come out with moist to dry crumbs. Try not to over bake the loaf. Let cool for at least 20 to 30 minutes. Slice up this baby up!