Next, the plantains. You can boil the plantains with or without removing the outer covering (peels). If you want to keep the peels on, cut the plantains into 1-inch circular pieces. This cut will take about 15–20 minutes to cook. Thicker slices will take longer to cook.
If you want to remove the skin, run the knife along the length of the plantain and cut about 1/8 inch deep into the peel. Peel the plantain with your fingers, then cut it into 1-inch pieces. I prefer leaving the peel on because removing it causes the plantain to absorb more water and become slightly soggy.
Place a large pot on medium heat, add water, and bring to a boil. Add the plantains and 1 teaspoon salt. Cook until they are entirely tender (check with a fork). Remove them from the heat and discard the water. If you left the peels on like I did, remove them after the plantain has cooled.
Keep the plantains in the same pot and add butter. Using a masher (or fork), mash the plantains until smooth. Add an egg, combine, and set aside.