How do the Spanish restaurants make plantains taste like candy?

I have tried buying ripe plantains and frying them in butter to a nice golden brown, but mine are just never quite as good as the ones I get in most restaurants. What is their secret for making them so sweet?How do the Spanish restaurants make plantains taste like candy?
Honestly and I am talking from experience because I grew up eating it, you have to buy VERY ripe plantains. Then slice them diagonally and fry until nice and hot. Then serve. You don't add butter or sugar, because the banana has enough sugar naturally. You're probably not buying the right plantains. Sorry.
You need to buy the yellow plantains and wait until they are ripe, although they may look black on the outside, you will notice that the inside is nice and soft, you slice them diagonally and fry them in very hot oil until golden.How do the Spanish restaurants make plantains taste like candy?
Boil the plantains, and then add butter/sugar afterwards. That's what I do.
They fry plantains with brown sugar