Place the sweet potatoes in a pot, and fill with enough water to cover. Bring to a boil, and cook until potatoes are easily pierced with fork, about 10 minutes. Drain well, place the potatoes in a mixing bowl, and mash.
Meanwhile, heat the 1 tablespoon of oil in a frypan over medium-high heat. Stir in the jalapeno, spring onions and garlic. Cook and stir until the vegetables are soft, about 5 minutes. Stir the vegetables, brown sugar and allspice into the mashed sweet potatoes in the bowl. Season to taste with salt and pepper.
Form the sweet potato mix into 12 slightly flattened cakes about 5cm in diameter using your hands or large spoons. Place on a plate.
Heat the 1/4 cup of oil in a frypan over medium-high heat. Place the sweet potato cakes in the frypan, four at a time, and cook, turning once, until golden brown on each side, 6 to 8 minutes. Add more oil if needed.