Here are the steps to take if you have trouble getting your fudge to set: The butterscotch layer will act slightly differently than the white chocolate layer because of the two different types of chocolates.  It’s important to boil the butterscotch layer for exactly 4 minutes over medium-high heat. I recommend setting a timer and adding in the butterscotch chips right at the 4-minute mark. You do not want to undercook this layer or it won’t set. The mixture should be thick, but still pourable. It’s easier to underset the butterscotch layer than the white chocolate layer.  It’s much easier to overcook the white chocolate layer, so only boil the mixture for 2 minutes.  For the white chocolate layer, don’t forget to take the mixture off of the heat before adding in the white chocolate chips. Stir until the chips are just melted and no longer, or the mixture will seize up and you will need to start again.  You can tell if your mixture has seized up if it becomes extremely thick and liquid releases out of the mixture.