This is a WONDERFUL all purpose chocolate frosting that is very easy to make. But, it does cool quickly, so it isn't the best choice for cupcakes. If making a 9x13, you can't go wrong. Put this on top of box mix cake, and everyone will think you did the entire thing from scratch.
In saucepan combine:
4 Tbs butter or marg
1 c. sugar
4 1/2 Tbs milk or half and half
Get to a strong boil over medium heat for one minute, not more.
Beat in 1/2 c. chocolate chips until fluffy - about double in size.
(I see no reason you couldn't substitute white chocolate or butterscotch chips)