Heat 3 tbsps (60g) of the butter and 2 tbsps (40ml) of the oil in large frypan on medium heat until butter is melted. Add half of the plantain slices; cook 2 to 3 minutes per side or until golden brown. Using tongs, transfer plantain slices to large bowl. Cover bowl with foil to keep warm. Repeat with remaining plantain slices. (Do not drain any of the remaining butter and oil left in the frypan.)
Heat remaining 2 tbsps (40ml) oil and 1 tbsp (20g) butter in same frypan on medium heat until butter is melted. Stir in garlic with a whisk. Remove frypan from heat. Whisk in remaining ingredients until well blended. Pour sauce over plantains in bowl. Coarsely mash until well blended. Serve immediately. Sprinkle with additional finely chopped coriander, if desired.
If any plantain does not peel easily, cut off the end of the plantain. Using a sharp knife, carefully make 4 lengthwise slits just to the flesh of the plantain on opposite sides of the plantain’s skin. Peel of the sectioned pieces of the skin to expose the flesh of the plantain. Cut each plantain into 1.25cm (½-inch) slices.
If not using plantains immediately, soak them in salted water to prevent them from browning. Drain and pat dry.
Ají amarillo paste is made from ají amarillo, a hot and fruity Peruvian yellow chilli. They can be found in South American or Latin markets or some gourmet supermarkets or online specialty stores.