Heat ½ cup (125 mL) butter and 6 tbsp (90 mL) oil in large skillet on medium heat until butter is melted. Brown plaintain sliced in batches; 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 skillet.)
Heat remaining 6 tbsp (90 mL) oil and 3 tbsp (45 mL) butter in same skillet on medium heat until butter is melted. Stir in garlic with a whisk. Remove skillet 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 cilantro, if desired.
• To peel plantain easily: cut off the ends of the plantain. Using a sharp knife, carefully make 4 lengthwise slits just to the flesh of the plantain. Peel off the sectioned pieces of the skin to expose the flesh of the plantain. Cut each plantain into ½-in (1 cm) 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 pepper. It can be found in Latin markets, online specialty stores or the Latin aisle of some supermarkets.
• For a delicious Peruvian-inspired meal, serve with Peruvian Chicken with Chile Sauces.