[bgill]

in a restaurant, it's "lots of cream, butter, salt and pepper held together with a little potato.."

And sugar.

[perfect stranger]

With mashed sweet potato, yes.

Not at all with standard mashed potato. Who told you that?

I have cooked for many restaurants in my time and so far NONE of them have used sugar in any mashed potato recipe as a specific ingredient.

Caramelized shallots are sometimes included to flavor the mashed potato, and a bit of sugar is used when you cook shallots like that.