The ideal plantain for this dish is one that is no longer bright green but not quite bright yellow with brown spots, but you can also make this dish with green plantains. That dish is called Mangu and it hails from the Dominican Republic and is savory, starchy, and delicious. You could also make this with a riper plantain for a sweet plantain mash. You can't really go wrong as long as you think of which level of savory or sweetness you desire before you start. That said if you are making this as a bed for the Cilantro Butter Shrimp the ideal flavor balance is somewhere in between ripe and green.
2 semi-ripe plantains ( see note above)
1 teaspoon salt
1 Tablespoons olive oil (you’ll need more for green plantains)
1/4 cup water
Cut the ends of the plantains, then run your knife lengthwise cutting through the peel but not the plantain. Peel the plantain by pulling open apart the long cut just made.
Cut the plantains in half lengthwise and remove the seeds. Cut the plantain again, this time into 2” pieces.
Place the plantains in a pot large enough to hold them and cover them with enough water to have at least an inch of water above them. Add the salt.
Boil the plantains until they are very soft. About 10-20 minutes depending on the plantain's stage of ripeness. As the plantain pieces cook, a white foam will form on top of the water. You can remove it with a spoon as you would when cooking dry beans.
When the plantains are very soft remove them from the water, place them in a medium bowl and carefully (they will be very hot) mash them right away. Add the water and oil as you go, and keep mashing until you have a smooth puree. Taste and adjust the salt.