Fill a large pot with water and heat over medium until boiling.
Remove skins from plantains. Plantains should be firm but with a bit of give. Slice each plantain into three pieces, then add to boiling water. Season with salt and cook plantains until fork tender, about 20 minutes. Meanwhile, in a bowl add butter and olive oil.
Add plantains to bowl and using a potato masher, mash down plantains and mix with oil and butter. Add hot cooking water a 1/3 cup at a time until a smooth mash forms.
Add lime juice and cilantro. Mix to combine. Season with salt and pepper to taste. Season with salt and pepper to taste. Serve hot.