Peel the plantains: the skin is so thick that you will need to cut a slit down the length of the skin. Make sure to cut no more than about 1/4″ (6 mm) deep (I did cut somewhat deeper than I wanted here, but it didn’t harm the final product).
Cut plantains into 1″ (25 mm) chunks. Place in a pot with chicken stock or juices from Chicken Ropa Viejo. The redness of the juices here are from the tomatoes used in the ropa viejo.
Bring to a boil. Cook for 10 minutes. Remove from pot with a slotted spoon and discard the juices.
Melt butter in the same pot, then add garlic. Saute while stirring continuously for 2 minutes or until they just start to brown. Add plantain chunks to the pot. Stir to coat, then continue heating to brown the plantain chunks slightly.
Smash plantain chunks in a molcajete (mortar and pestle) or place plantain chunks in a bowl and smash with a potato masher just enough to break the skin. Don’t work so much that it turns to the consistency of mashed potatoes.
Mix plantain mash with Chicken Ropa Viejo. Blend evenly, but try not to break up the plantain chunks.
Form using a ring mold. Garnish with cilantro if desired.