How much powdered sugar to use for frosting?

When I make a powdered sugar frosting, I start with equal parts butter and sugar (by weight), and add sugar or liquid to adjust. I've had the same ratio of butter/sugar behave differently on different days with different brands so I've just learned how to gauge for myself. Just know that when you're adding liquid, you'll need more sugar.

What I'm confused about is, when I'm adding melted chocolate to frosting, it often has the opposite effect and provides a bit of stiffening. If you're concerned about the amount of sugar, add the candy to a frosting made with less sugar and then add the sugar if you need it. The sugar is the most easily adjustable part of the recipe, so its better to hold back than it is to alter the full recipe with liquid.

@Kay, I think the reason there is 5 cups of powdered sugar is because you'll be adding 3 melted candy bars. That extra cup of sugar is probably added to make the frosting stiff enough to not be running off of the cake. Ordinarily, the frosting recipe has just the butter, so you could possibly add less sugar and it would still be stiff enough to frost the cake. But since you'll be adding something most liquidy, you will probably need that extra cup of sugar to insure that the frosting won't run off the cake.

If you do try to cut back on the sugar, you should add enough to get a consistency that is firm enough to frost and not be runny.

Good luck! Your grandson is very lucky!