I had the same problem with the frosting until I turned up the heat. I'm an experienced cook and prefer to weigh my ingredients.  With light and dark brown sugar the weight is from 5 oz to 8 oz per cup.  I would appreciate being given the ingredient weights, because it can make a huge difference.  Also, when I bake I always under time.  It's a good thing I did this time. The cake took less than 25 minutes. I'm a little concerned it will be dry. LM

December 2, 2007