Place white chocolate in a medium bowl and melt in a microwave by cooking in 30 second intervals, stirring, and repeating.
Add frosting to the bowl and stir until blended.
Spread into prepared pan.
Decorate top if desired.
Place in refrigerator for at least an hour or two to set.
Use parchment paper to remove from the pan and cut into squares.
In this size pan, the fudge pieces are about 5/8″ thick. If you want thicker, make a double batch.
Notes
If you're using white chocolate, keep a close eye on it- you want to heat it until it's just melted. If you heat it until it's really runny, your fudge might not turn out correctly.