Peel the plantains and cut each one into 1/2-inch thick rounds and set aside. Heat the oil and butter together in a large deep frying pan over medium heat. Add the plantain rounds, being careful not to overcrowd them. Allow them to fry gently for about 1-2 minutes or until they are golden brown on one side, then gently turn them over. While the plantains are frying make the garnish if using: Place the sour cream, powdered sugar and vanilla in a small bowl and mix well. Set aside. Fry the plantains until brown on the other side and remove from pan, placing them on a paper towel lined plate. Repeat until all the plantains are cooked. Sprinkle with cinnamon and sour cream garnish if using. Sprinkle sour cream garnish with a little extra cinnamon. Serve warm.