Really Really Good Chocolate Frosting

I will be the first to admit that our go-to birthday cake is lovingly known in our house as "puddin' cake" which consists of a box cake along with a box of vanilla instant pudding. It goes against everything I believe in to use a "box" anything but damn! that cake is good-- moist and unnaturally yellow in color. However, I draw the line at canned frosting... it's just wrong and not good and wrong. Please try this recipe- it is so easy and only uses one pan!  You'll never return to the canned stuff, I promise!!

1 1/2 sticks unsalted butter
4 oz. unsweetened chocolate
3 T cocoa powder (nothing fancy- just regular cocoa)
3 1/2 to 4 cups powdered sugar (depending on how sweet and soft you want the frosting)
3/4 cup sour cream

In a large enough saucepan over low heat, melt the butter along with the chocolate and cocoa powder, stirring with a heatproof rubber scraper. When it is smooth, remove from heat and whisk in most of the powdered sugar; I only sift if forced to at gunpoint but if you want to do it- feel free). Whisk in the sour cream and then the rest of the powdered sugar.  Whisk until the frosting is smooth (enough). Use right away.