I just had this problem when I was making scones, only the butter was in the freezer! When I've tried microwaving to fix this, I usually end up with butter that's soft on the outside but still firm on the inside.
I googled and came up with this solution, which worked great: Cut the butter into very small pieces. As you expose more surface area, it softens easily. HTH.
The trouble with eating Italian food is that five or six days later you're hungry again. ~ George Miller