Rocky road seized up?
Every recipe I looked up said pretty much the same thing. You have to make sure that the melted chocolate does not touch any water or steam, otherwise the chocolate will seize up.

Recipe: "Melt white chocolate in a microwave safe bowl, in the microwave by heating on high for 1 minute, stir with a metal spoon then heat for bursts of 30 seconds stirring in between bursts until melted. Make sure that anything that touches the chocolate (bowl, spoon etc) is really dry otherwise the chocolate will seize up (so don't use a wooden spoon coz the wood may have moisture in it!)"

Recipe: "Melt the chocolate in a bowl placed over a pot of gently simmering water. Don’t let water or steam come into contact with the chocolate as the chocolate will seize. Stir until melted."

I found this recipe (with butter and syrup) and it uses chips instead of the normal chocolate: "Before you make your shopping list, please note that there are three measures of white chocolate - I used white chocolate melts for the top because they are whiter and easier to melt than normal white chocolate, which can seize up if you don't treat if very gently"

There are many more similar recipes and all of them mention the water not touching chocolate bit to avoid seizing.